I know people who work out every single day and seem to manage just fine, but most people take one day off a week to relax. This helps your body recooperate and prevents you from feeling worn down, giving you more energy in the gym so you can hit those heavy pounds. For instance, I relax on Sunday and have my problem area (chest) on Monday so I can focus on this area. I know that if I didn't rest the day before, my chest workout wouldn't be as good.

Personally I don't take a day completely off however. On the weekend I don't lift but do cardio, which still keeps me active but gives my body a rest from lifting. Its all a matter of what your comfortable with, but I think one day of rest is a week is good for the body;