Rainbows are seen when raindrops falling in the distance bend and bounce sunlight back towards your eye.
Secondary rainbows are higher up in the sky and coloured in reverse.
Supernumerary rainbows are the banded fringes seen on the inside edge of a rainbow.
Other bows can be made by the Moon, or in fog.
Interesting sky lights such as mock suns, halos and sun pillars can be created by ice crystals.
Sources of further information about rainbows