A member of the nightshade family (along with aubergines, peppers and chillies), tomatoes are in fact a fruit, but their affinity for other savoury ingredients means that they are usually classed as a vegetable.
Tomatoes originated in western South America, crossed the Atlantic to Spain with the conquistadors in the 16th century, but only finally caught on in northern Europe in the 19th century. Today they're one of the most important ingredients available, and are especially indispensible in Mediterranean cookery. The skin, flesh and seeds can all be eaten, but the green leaves are toxic, so should always be discarded.
The number of varieties run into the thousands, and they vary in size from the huge beefsteak to tiny cherry tomatoes, but most have a sweet, gently tangy flavour and are good both raw and cooked.