As taken from Wikipedia: http://en.wikipedia.org/wiki/Caviar
Caviar, according to the Food and Agriculture Organization, is a product made from salt-cured fish-eggs of the Acipenseridae family. The roe can be "fresh" (non-pasteurized) or pasteurized, with pasteurization reducing its culinary and economic value.
Traditionally the term caviar refers only to roe from wild sturgeon in theCaspian and Black Seas (Beluga, Ossetra and Sevruga caviars). Depending on the country, caviar may also be used to describe the roe of other fish such as salmon, steelhead, trout, lumpfish, whitefish, and other species of sturgeon.
Caviar is considered a luxury delicacy and is eaten as a garnish or a spread. In 2012, caviar sold for $2,500 per pound, or $3,000 to $5,500 per kilo.
According to the United Nations' Food and Agriculture Organization, roe from any fish not belonging to the Acipenseriformes species (including Acipenseridae, or sturgeon sensu stricto, and Polyodontidaeor paddlefish) are not caviar, but "substitutes of caviar." This position is also adopted by the Convention on International Trade in Endangered Species of Wild Fauna and Flora, the World Wide Fund for Nature, the United States Customs Service,and France.
The four main types of caviar are Beluga, Sterlet, Ossetra, and Sevruga. The rarest and costliest is from beluga sturgeon that swim in the Caspian Sea, which is bordered by Iran, Kazakhstan, Russia, Turkmenistan and Azerbaijan. Wild caviar production was suspended in Russia between 2008 and 2011 to allow wild stocks to replenish. Azerbaijan and Iran also allow the fishing of sturgeon off their coasts. Beluga caviar is prized for its soft, extremely large (pea-size) eggs. It can range in color from pale silver-gray to black. It is followed by the small golden sterlet caviar which is rare and was once reserved for Russian, Iranian and Austrian royalty. Next in quality is the medium-sized, gray to brownish osetra (ossetra), and the last in the quality ranking is smaller, gray sevruga caviar.
Cheaper alternatives have been developed from the roe of whitefish and the North Atlantic salmon. In the wake of overfishing, the harvest and sale of black caviar was banned in Russia in 2007 but resumed in 2010, limited to 150 kg (330 lbs).