Cameos and References can be any of the following:
Cross-Series: When a character or object from one game or game series makes an appearance in a completely different game or game from another series.
Specific Reference: A VERY specific reference in one game to another in the series. Be careful with these! Reference must be extremely specific to one event or place or character. Reoccurring ideas, characters, places, enemies, etc., do not count as cameos!
Character Cameo: When a character or object from one game makes a brief or subtle appearance in another game in the same series and is not directly related to the gameplay or plot in some way.
Famous Person: A celebrity guest appearance or character that strongly resembles a famous (real) person.
Music: When a song that was not specifically written for a game is used as part of the game's music.
Movie: A very specific reference to something in a movie.
TV Shows: A very specific reference to something in a TV show.
Commercials/Products: A very specific reference to something in a well-known TV commercial, or a well-known product. Please note that ads placed in games for modern systems do not count. X-Company will not get a credit in the database for paying X-gamemakers to put ads for their products or services in the game. (I will still count these for 16-bit systems and everything before them because they were much more rare back then.) Spoofs of companies, products, and commercials in modern games are fine, though.
Mythology: A very specific religious or mythological reference.
Console: When a game console or part of its hardware appears within the game.
Literature: A very specific reference to something in a book.
Programmers: A reference to the game's programmers or the company that made it. NOTE: Must be included within the game, somehow, not part of the opening or closing credits.
Real Place: When a location in a fantasy setting is clearly based on a real-world location. Be careful with this one, games that take place in the real world do not count.
Art/Artists: Something based on a famous artist or famous work of art.
Comics: Something based on a character in a comic book or comic strip.
Miscellaneous: A reference to something that doesn't really fit into any other category.