White light is comprised of the complete spectrum of colours that make up the visible part of the electromagnetic spectrum (red, orange, yellow, green, blue, indigo, violet). Each of these components has a different wavelength and frequency. As white light moving through air enters a new medium the light wave slows down (this is what causes refraction). Further to this, each of the different colours slow down to different degrees. This results in each of the colours refracting at slightly different angles. When white light enters a prism as shown below the refraction of the different colours is exaggerated and the different colours are split. This splitting of white light into the different colours is called dispersion. Dispersion is responsible for rainbows that we see in the sky due to the refraction of light through raindrops in the atmosphere.
The blue/violet end of the spectrum slows down the most so it refracts the most, whilst the red/orange end of the spectrum refracts the least.