What is red-shift and how does it work?

When we see light, the colour of light that we see is determined by the wavelength of the light. Violet light has the shortest wavelength, and red light has the longest wavelength. Red shift happens when we see light that is 'redder' than when the light was emitted. This means that the wavelength of the light has increased while the light was travelling. Most physicists think that this happens because space was expanding as the light was travelling through it, and so red-shift is used as evidence that the universe is expanding.