This is usually seperating out the two main aspects of getting to a bodybuilders physique. cutting is losing fat, bulking is gaining muscle.

what all the supplements don't tell you is its really hard to gain muscle and lose fat at the same time. most bodybuilders will cycle through 12 week time periods with specific goals. Say you have a competition 6 months away, you would bulk for the first 3 months, then start cutting, usually through a combination of carb controlling and portion controling,

The trouble is also in the fact that during bulking, you add some fat, and during cutting, you lose some muscle.

Just in general, most people have a hard time losing weight in the 3rd month of a diet, and the last few pounds are also the hardest. bodybuilders get down to 6-9 % bodyfat, This is a hard place to be for a long period of time, so most have a "pre-contest" weight, a off season weight, and a contest weight goal.

something to lok up that also might help is the term periodization, its what a lot of pro atheletes use as a training regimin.