Capers are the flower bud of a bush native to the Mediterranean and parts of Asia. They range in size, but are all small. They are not eaten fresh, but are pickled in vinegar brine. They are also available packed in salt, which you soak and rinse once or twice before eating.
Capers are used in a number of classic French and more modern sauces and dressings (rémoulade, ravigote, Tartare, Caper-Lemon Butter, Creamy Dijon-Caper Dressing). They go well with seafood and mutton, and as you yourself know, garnish salads.