"If I just do more 'cardio' I'll lose weight."
"I can get rid of my cellulite if I do more 'cardio.'"
And this may be true - Stairmaster, treadmill, elliptical machines, stationary bicycles - Moderate to intense use of this equipment will in fact raise your heart rate, improve your cardiovascular fitness and burn a few calories. BUT, you can spend hour upon hour on these machines and never achieve muscle definition. On top of that, when you are doing traditional 'cardio,' you are only burning calories while your heart rate is elevated. This means that by the time you reach your car your metabolism will already be back to where it was before you walked into the gym.
If you weight train, you can burn serious calories throughout your workout AND, by increasing your muscle mass (lean body mass) you will burn more calories at rest. Yes - you heard me right.
For example, if you add a single pound of new muscle to your body (and you maintain it) you will burn 50-100 more calories (depending on your metabolism) EVERY DAY while sitting on your ass watching TV - as long as you don't have your face buried in a bag of potato chips or a gallon of ice cream.
It would seem that through some nutty evolutionary mechanism, our bodies would prefer to burn fat rather than muscle for energy when given the choice. By weight training, you are making your body choose and it will always take the most efficient path.
Want to learn more? Read this article from American Fitness magazine.