Caviar, from Persian word Khāg-āvar, is salt-cured fish-eggs of fish of Acipenseridae family. Nevertheless, it may also be used to indicate the roe of other fish, such as salmon. In fact, on the market, you can find several types of Caviar, each of them taking the name from the fish the roe come from. The most common are beluga (or European) sturgeon, ossetra (or Russian) sturgeon and sevruga (or starry) sturgeon.
In cooking, Caviar is considered as a real delicacy, and is actually quite expensive. Anyway, you can use it, for example, just for special occasions. It is great for cold dishes, or as a garnish, like in sole with celery and potatoes. Just remember to serve it cold, and within a few hours from the purchase moment, as it is extremely perishable.
Fun fact: Caviar has a really high content of vitamin B12, but is also high in cholesterol and salt.