If I had to choose one exercise to teach and do the rest of my life I would choose the squat. The squat? Yeah, the squat. As lowly as it may seem the squat gives the most return for the effort. What in the world does that mean? Well, it hits the most muscle for the movement IF it is done properly AND through the full ROM (range of motion).
Heels down, Chest up, Knees out, Hips below Knees, and Neck in a nuetral position. The arms and upper back are in isometric contraction. The squat has several variations that can be done by everyone. The Air Squat, just your bodyweight here, no bar. Sounds easy, well crank out 100 as fast as you can. Still easy, go for 500, fast but maintain form and full ROM. Yeah, burns doesn't it. Keep going, don't be a meow.
The Back Squat is what every person who is starting a lift program should be doing 3 days a week for months. Men, women, children, everyone should be doing the back squat and adding weight every workout. Stay away from the Smith machine, it is just going to hurt you and you ain't going to get strong using it. At it's worst it forces you into a plane of motion that isn't natural and the pure squat gets the blame for your jacked up knees. At it's best the Smith machine is good for hanging your laundry on to dry. Squatting in the Smith machine isn't squatting, I don't know what it is but it ain't squatting.
Have you done the Front Squat? Want to tighten up you flabby arse? Then the front squat is for you. It removes the hamstring from the movement and forces the glutes to bear the burden of the lift. Stop thinking that step class and hours on the eliptical are going to rid you of the scourge of cellulite and control your diet and do the front squat. Did I mention that it helps to develop a stronger C&J?
Overhead Squatting is the king of the squats. Try it sometime, but beware, start with a very light weight. How is your balance...flexibility...intensity? O'yeah, if your hips aren't below your knees then it don't count. Are you ready....go for 15 repititions with your bodyweight. Let me know how you do.
Read Coach Mark Rippetoe's book Starting Strength 2nd Edition Basic Barbell Training to get a great breakdown on the Squat. Better yet attend one of his certifications. If you are serious about your health and performance then you should have this book in your library. Of course you can continue to do curls in the Squat rack and wonder why you don't see any progress.
Do the Squat and buy the book. You won't regret it!