Definition of black caviar: Fish roe that comes solely from fish of the Acipenseridae family, also known as sturgeon.
Definition of red caviar: Fish roe that comes from a salmonid; any fish belonging to the Salmonidae family.
There are many ways to categorize the various types of caviar and fish roe. The simplest, most traditional and widely used method of categorization is the black caviar/red caviar system. This separates caviar and fish roe based loosely on the color of the eggs.
This classic technique is not inclusive to all types of fish roe, but defines black caviar as roe from a sturgeon and red caviar as roe from a salmonid. Fish roe that is from a sturgeon is considered black caviar because the eggs are commonly darker in color. Red, orange and even yellow fish roe usually from salmon (sometimes trout, whitefish, etc) is known as red caviar despite not actually meeting the traditional definition for caviar.