Capers are a key ingredient in Mediterranean and French food. They have been part of Mediterranean cuisine and culture for hundreds of generations. There was even a time when these were used as a kind of currency.
Capers are small buds which are harvested from caper bushes before they are ripe, then dried under the sun and brined in vinegar. Caper bushes are easy to grow and in fact thrive in sandy soil where most plants would die. It is possible to see caper bushes on the sides of broken roads. However, capers have little flavor on their own. They must be harvested at a particular point in the growth cycle and then processed. They are dried under the sun, pickled in a brine solution, and then bottled (sometimes in pretty decorative jars). The process is quite labor intensive.
Capers are never eaten on their own. The flavor is far too strong, and in fact, many chefs will recommend actually rinsing them out in water before even adding them to a dish. A little goes a long way, so most recipes will only call for a small teaspoon or so. Capers are often used in making sauces, or adding flavor to meats. They are also used in salads and pasta dishes. They are quite ideal for fish sauces and fish marinades.
Capers should not be confused with caper berries. Caper berries are left to ripen longer on the stem, and are much larger (similar to olives). They can be eaten as a snack, or sometimes sprinkled on salads. In general, though, when recipes call for capers, they mean the smaller, pickled ones.