Olives are the little fruits of the olive tree. They contain a stone, surrounded in the soft flesh of the olive fruit. They are most common in the Mediterranean, but have been grown and traded right across the Med, Middle East and North Africa for centuries.
There are a whole variety of olives all differing in size, colour and (most importantly) flavour. The taste of olives can be described as savoury, salty, bitter and buttery, but the flavour does indeed massively change depending on the variety you're eating. This is also the key to making full bodied, flavoursome olive oils.