A color wheel is an easy way of showing the primary, secondary, and tertiary colors and their relationship to each other. In its simplest form, it will show concentric bands of color.
The inner circle split into the 3 colors. These are the primary colors, Red, Blue, and Yellow.
That circle is then surrounded by a 2nd circular band of colors. These are the secondary colors that are mixed by using equal amounts of 2 of the 3 primary colors. This is explained more in our article about secondary colors.
The 3rd band is what we call the tertiary colors. These can be represented as they are here in blocks of color, or they can be painted as a slow, gradually changing color that is created by varying the amounts of the colors you are mixing.
In principle, every color can be mixed simply by varying the amounts of the three primary colors. Our article on primary colors (see link above) explains this in more detail.