The additive primary colours are red, green and blue (often shortened as RGB). When added together in equal amounts at 100% intensity, white light is created. When added together in different proportions, all other colours can be created.
Red + Blue = Magenta
Red + Green = Yellow
Blue + Green = Cyan
Red + Blue + Green = White
When speaking of primary colours, confusion is often created because there are two different types of primary colour. Additive primaries describe the colours we see in the real world, created by light. Subtractive primaries (cyan, magenta and yellow) are used in print media to mix pigments.
The subtractive primary colours also happen to be the complementary colours to additive primaries. These are sometimes known as light secondaries or secondary colours, being the colour that is crreated when two primary colours are combined.