The Truth About Muscle Building
You can target what muscles you build, by doing exercises that work that specific muscle (unlike fat loss which cannot be targeted by body part/region). For example, if you want a rounder bum, remember it's a muscle with fat on it. You can lose the fat off it (well, that area and everywhere else at the same time) and build the muscle so it bulges out more.
Exercise is muscle shortening. Muscles can pull themselves together or flex, then relax. They work in pairs - one pulls apart one way and its pair pulls it back.