The Color wheel is made up of Primary, Secondary, and Tertiary colors. Depending on who's color wheel you're looking at, the Primary Colors typically consist of Red, Blue, and Yellow. Secondary colors are the biproduct of adding two primary colors: Red+Yellow=Orange, Yellow+Blue=Green, and Blue+Red=Purple. The Tertiary colors are the biproduct of adding a Primary and a Secondary color together. Below is a representation of a simple color wheel.
Of Course, there is a wide variety of 'colors' to choose from at your local paint supply store. Most colors derive from one of the hues above and mixed with other hues, white, and black to give varying values.
Believe it or not, color DOES make a difference in your space. Not only can it help to make a room feel larger or smaller, but it can also stimulate certain moods.