Known also as roe, caviar is simply fish eggs harvested from the sturgeon, salmon and other fish. Some types are quite rare, such as beluga, leading to bans on imported caviar of specific types in countries. To add insult to injury, the task of successfully harvesting eggs is painstakingly hard as the sturgeon must not come to any fear, if so, a chemical will be released which spoils the taste of the eggs.
Once the eggs are harvested, a fine delicacy has been crafted. Many think it is only a rich person's food, and in a way, it is. The unique taste of caviar is often an acquired taste at that. Caviar shouldn't be eaten alone to fill you up, this will burn a hole in your pocket. The reason it is eaten, is to enjoy the flavor alongside a meal, perhaps as a finisher. In a way you could say its an expensive lollipop. You really don't eat them to fill you up, but just to enjoy the taste. I, myself have only eaten caviar very few times. It is something to enjoy moderately.
Black caviar has a very peculiar flavor, it is usually less salted. The larger the grain of the black caviar the more expensive it is. the smaller the cheaper.
Red caviar is more salty and more fishy in taste, it is somewhat slightly reminiscent of lox, just slightly. Lets say if you put lox in a processor , kind of comparable. Red caviar is saltier and not as "fine" tasting, it is not as distinct as black.