What's the difference between "roe" and "caviar". Wikipedia has two different articles, however, the difference is not explain in the either one.
From an article on “What is the Difference Between Caviar and Roe?” at www.culinarylore.com:
They refer to the same thing: fish eggs. However, the term roe refers to the fish eggs (or male fish sperm) themselves while caviar is roe that has been salted or "cured" and then placed in tins for storage and aging. In the United States and Canada, any product that is only labeled caviar must come from sturgeon roe. If it contains the roe from any other fish, such as whitefish, it must be labelled with the name of the fish, such as whitefish roe, salmon roe, carp roe, trout roe, etc. The roe of shellfish is also used. In Europe, however, the term caviar is reserved exclusively for the roe of sturgeon. Roe from other fish must be called caviar substitute.
Here is a pictorial comparison of them, with salmon eggs on the left and sturgeon caviar on the right: