Cardiovascular exercise ("Cardio" for short)  is    more important  for your general health than weightlifting. More important than that to  most   people is that 
cardiovascular exercise helps you lose fat and look better! The quickest way to look like you have packed on 10lbs muscle is to lose 10lbs of fat, its a really cool bodybuilding illusion. It can take a years to add muscle but in a month you can 
look  more muscular just by losing weight, it's because bodyfat makes the  body round and shapeless which hides the muscles. Just check my video  comments over the last 6 weeks, I dropped 7lbs of bodyfat with 
carb cycling  and everyone keeps commenting how much more massive I look even though I  haven't gained an ounce of muscle. So if you don't do cardio for your  health, do it for your appearance!
Cardio is a confusing topic to many so I cover it in detail here. I discuss 
how much cardio to do, 
when do cardio, and how to know 
what is good cardio.
The Importance of Cardio
If you don't have time to do cardiovascular exercise,    then  skip weightlifting so you can do it - its that important.   Cardiovascular exercise should not be boring,   it should be fun!  If  you have to do cardio at the gym then find a good   book, listen to the  radio or books on tape - find a way to make it interesting   and  enjoyable.    Better yet, find a sport you love and do it with a spouse  or friend and the time will fly by.  Walking,   jogging, cycling,  soccer, skateboarding, or whatever excites your passions.  If you don't    have much time then jog to the gym, walk you kids to school, walk your  dog, or   start using your bike for transportation.  If you don't  belong to a gym, and   don't have a thousand dollars for a good  stationary cardio machine, buy a   jump-rope!  Jumping rope is an  amazingly intense aerobic workout, in fact,   you will probably have to  start with just a few minutes and work up to 20-30   minutes a day.   Jumping rope is not only an amazing aerobic workout but is   great at  increasing coordination.  Contrary to popular belief, jumping rope   is  not monotonous either if done properly and takes a lot of  concentration.    One drawback is that you need a room with high  ceilings to jump rope or you need   to do it outside.  Here are some  great reasons to do cardio:
-  To lose fat! - want 6-pack abs, only way is to lose fat thru cardio and    nutrition.
-  Cardio can improve your mood and significantly reduce  depression.  Its been    proven by study after study, cardio can make  you happier!  
-  Perform better at school or work!  Daily cardio reduces  mental fatigue and    gives you more energy. A recent study from the U  of G showed that daily    cardio reduces fatigue by 65%
-  Tons of other health reason -         -  reduce risk of     heart disease
-  reduce blood     pressure
-  reduce risk of     diabetes
-  live longer - 2hrs     longer for every 1hr exercise
 
How much cardio?
If you are happy with your   body fat percentage and just   want to keep your heart healthy then follow the 
American Heart Association's    guidelines and do 30 minutes of  light exercise (walking, gardening,  etc)   a day.  If you want to lose body fat then try and do 30-60  minutes of   strenuous cardio (swimming, running, fast biking) every  day.
If you are lacking in time then   find some part of your week  where you can bike for transportation instead of   driving, to work, the  gym, or the grocery store perhaps?  Time sitting in   your car is not  only expensive but it is a waste of time, try and find some way   of  biking instead of driving the car.  If you are overweight enough that  you   don't feel comfortable   biking then walking is a great option.   Try walking as transportation,   beats wasting time sitting in traffic.
When should you do your cardio?  
There is a lot of confusion about when to do cardio, I get asked  this question all the time!  As many of you have heard, doing cardio  before you eat in the morning burns off more calories so is that the  best time?  Lots believe that the right time to do cardio is before a  weight workout to provide a warmup but just as many believe that the  right time is afterward.  Who is right?  None of them! All of them!
The right time to do cardio is when you are most likely to do it!   Cardio is very important and its 100x more important that you actually  DO your cardio than when to do it!  Lets look at some of the pros and  cons of doing cardio at various times.
Doing Cardio And Weight Workouts Separately
Its optimal to do your cardio and weight workouts completely  separately, that is, with four or more hours of rest between them.  This  way, neither workout interferes with the other and you can give each  110% effort.  For those of us who workout at home, this is easy to do.   For those who go to gyms, its not as easy if you depend on the  stationary equipment for your cardio because then you need to make a  second trip to the gym.  Now lets look at the non-optimal compromises.
Doing Cardio Before Breakfast
Yes, studies have shown that you burn off more calories if you do  cardio before breakfast, but its a lousy time to do cardio in my  opinion because you wont do your cardio for nearly as long.  Who can  focus on cardio when your stomach is growling and you feel faint from  lack of food?  Not me!   Besides, doing cardio before breakfast isn't  the most convenient time for most folks. 
Doing Cardio Before A Weight Workout
The fans of doing cardio before a weight workout point to its  ability to help warm up your muscles to prevent injury, true it does  this.  For those who significantly overweight or are fitness beginners,  walking or light jogging is the recommended cardio. For these people  doing their walking or light jogging before their weight workout is  perfect, it warms them up and doesn't interfere with their weight  workout.  On the other hand, intermediate to advanced athletes need to  push themselves much harder in their cardio workouts - they get their  heartrate higher and keep it elevated for longer.  Cardio workouts of  this intensity will exhaust the body to the point where the weight  workout will be lackluster at best reducing potential strength and mass  gains.  If you are an intermediate or advanced athlete whose time is at a  premium and don't feel its important to add more muscle mass then doing  cardio before the weight workout is ok, otherwise find a better time to  do cardio.
Doing Cardio After A Weight Workout
Doing cardio after a weight workout is my personal favorite, its  not optimal but its a very good use of time if you are doing your cardio  at a gym because it eliminates the need for a second trip to the gym.  I  always find that even after an intense weight workout, I still can give  cardio the intensity it needs.  One thing I always do though is make  sure to eat a small meal after lifting weights before starting the  cardio.  Meal timing is important and its important to get a meal right  after weight lifting and waiting till after you do 45min of cardio,  shower, and get home is too much of a delay. If you do your cardio after  your weight workout, you need to find some alternative way to warmup  your muscles before the weight workout, not a big deal.
Cardio Is NOT Boring!
If you find cardio boring then you are doing something wrong and  you need to fix it.  How can you do this?  Well, try doing your cardio  outside in the real world if you can.  Go for a bikeride thru the park,  go jogging alongside the river.  Join a soccer team or play ultimate  frisbee, team sports have a way of making your cardio time fly by.  If  you are stuck indoors because of weather, you can still make those  dreary stationary machines exciting.  Watch your favorite TV show while  you are doing cardio or read your favorite trashy magazine.  Another way  to make cardio engrossing is to download lots of interesting podcasts  or music to your iPod/iPhone.  An 
iPhone remote can make your iPod/iPhone much easier to use while doing cardio and makes it much less likely you will drop it.            This is your chance to get away from the stress of everyday life and have fun!      
What is   "good cardio"?     
Its   clear that there is a lot of confusion about cardio, its  really quite simple.    Lets start with the basics.  The purpose of  cardio is to elevate your heart rate   and keep it (in the purple zone)  there for at least 20 minutes.   How   elevated does your heart rate  need to be?  Depends on your goal, your age, and   your health.  If you  have medical conditions, ask your doctor what is   appropriate for you.   If you are in good health, you can use these charts from   the AHA    
           | Age | Target HR Zone             50–85 % | Average Maximum             Heart Rate             100 % | 
           | 20 years | 100–170 beats per     minute | 200 beats per     minute | 
           | 25 years | 98–166 beats per     minute | 195 beats per     minute | 
           | 30 years | 95–162 beats per     minute | 190 beats per     minute | 
           | 35 years | 93–157 beats per     minute | 185 beats per     minute | 
           | 40 years | 90–153 beats per     minute | 180 beats per     minute | 
           | 45 years | 88–149 beats per     minute | 175 beats per     minute | 
           | 50 years | 85–145 beats per     minute | 170 beats per     minute | 
           | 55 years | 83–140 beats per     minute | 165 beats per     minute | 
           | 60 years | 80–136 beats per     minute | 160 beats per     minute | 
           | 65 years | 78–132 beats per     minute | 155 beats per     minute | 
           | 70 years | 75–128 beats per     minute | 150 beats per     minute | 
     
   http://www.americanheart.org/presenter.jhtml?identifier=4736
So if   you are wondering if a particular activity is good  cardio, its simple to find   out.  Do it for a minimum of 20 minutes  then measure your heart rate (15sx4) and   compare it to the above  chart.  If your heart rate is in the target zone, then   its good cardio  - simple as that.   With many forms of cardio, like cycling or    walking, its easy to get lazy and just lolli-gag along.  Its important  that you   monitor your heart rate during the cardio to make sure that  you are not slacking   off.  Just a note, walking has the lowest dropout  rate of all forms of cardio -   probably because its so enjoyable.
OK,   now lets talk about cardio equipment. I get lots of  questions asking if a   particular kind of cardio equipment is good.    First of all, I don't recommend   buying ANY equipment.  I'd much  rather see you get exercise outdoors by cycling,   jogging, or playing  soccer, its cheaper and more enjoyable.  If you have to   workout in  your home, I would recommend a cheap jump rope.  Having said that, if    you are determined to spend money on equipment for indoor use, here is  how you   can tell if a piece of equipment is good.  
First   of all, good cardio equipment is very expensive and  typically costs over a   thousand dollars  and has three main  characteristics
-  It gets your heart rate into the target zone
-  adjustable speed, very impt
-  sturdy = heavy, never seen anything sturdy under 50lbs most well over a    hundred
-  sufficient adjustable    resistance 
OK,   some common questions I get about cardio.  Many people seem  to be looking for   shortcuts to cardio.  What if I sprint, can I just  do 1 mile?  NO! You must do   your cardio a MINIMUM of 20min, preferably  40-60min.  What if I do interval   training, can I just do 15min - NO!   You must do your cardio a MINIMUM of 20min,   preferably 40-60min.  I  get questions about interval training all the time so   lets talk about  that a moment.  Interval training is alternating bursts of speed    followed by short rests, for example in running you might sprint for 60s  and   walk for 30s.  Studies have shown that exercising in this fashion  can burn off   more calories than steady cardio.  If you want to do  interval training
-  you still need to do it for a min of 20min daily
-  don't rest so long that your heart rate drops below the target heart rate zone    (purple)
-  Be mindful of injuries, make sure to stretch - you are much more likely to    get injured sprinting than 
Wow,   I'm really groggy myself.  Time for me to do cardio before I fall asleep.
Some   important references:
 http://www.depression-help-for-you.com/exercise-and-depression.html
 http://www.psychosomaticmedicine.org/cgi/content/abstract/69/7/587
 http://www.sciencedaily.com/releases/2008/02/080228112008.htm