There are 6 main kinds of black caviar:

Beluga caviar has large eggs (up to 3.5 mm) and can have platinum, dark gray, silver or black color. It does not have a specific smell but may have a light nutty aftertaste and very delicate flavor.
Kaluga caviar has large eggs (up to 3 mm) and can be dark, gray or silver, tan or black. The taste is similar to beluga caviar but less pronounced.
Sturgeon (Oscietra) caviar has medium size eggs (up to 2.5 mm) and gray, slightly yellowish or brown, dark bronze or black color. Taste is a little sharper than beluga with hints of a sea and iodine, sometimes it has a wonderful nutty flavor.
Sturgeon (Sevruga) caviar has small eggs and platinum, dark gray or black color, it often has quite a lot of fat. Taste is light but intrusive.
Starlet caviar: the eggs size is average between Osetra and Sevruga, a color varies from dark gray to black. Taste is delicate.
Сaviar of sturgeon hybrids – taste and appearance depend on the individuals crossed.