A colour wheel is a visual representation of colours arranged in the shape of a circle or wheel. This circle has the primary, secondary and tertiary colours in it.
Primary colours are colours that cannot be obtained by mixing any colours. They are red, yellow and blue.
Secondary colours are colours that are obtained by mixing primary colours. For example the three secondary colours are orange (red+yellow), green (yellow+blue) and violet (blue+red).
Tertiary colours are obtained by mixing adjacent primary and secondary colours. Blue-green is an example of this group.