The great thing about tabletop roleplaying games, to the gamer, is that they're open-ended. Unlike their electronic descendants, they usually don't rely on a single in-built storyline, but allow the Game Master to implement an indefinite number of scenarios he or she may imagine.
The bad thing about tabletop roleplaying games, to the publisher, is that they're open-ended. After all, once a player has bought a copy of your game, they can use those rules to implement an indefinite number of scenarios they've imagined... for free.
Thus, to a large degree, game publishers are dependent on continual expansion of the game with new material outside the basic rules of play. These can include:
- Worldbooks: Not every Game Master may feel comfortable with—or capable of—the work involved in detailing their own world to set adventures in. These books provide ready-made setting material, either in whole or in fragments (e.g. a single city which might be inserted into any general world). Licensed properties may be covered in this type of sourcebook; for example, Big Eyes, Small Mouth does worldbooks for many popular Anime, while Rifts is well past it's twentieth World Book just talking about the Earth. GURPS has had several of these as well, mostly in the second and third editions.
- Rule expansions: Some games presume that not all rules are created equal. For instance, while battles between large armies are often found in epic fantasy, a game group wouldn't necessarily need them. In cases like this, the publisher breaks out the lower priority rules into a separate book from the original rulebook. For example, almost all GURPS sourcebooks include breakdowns or even modifications to the core rules to make the parts of there setting that much easier in game terms, while Warhammer 40,000 Apocalypse features rules and tips for large-scale battles with your entire miniatures collection.
- Splatbooks: A sourcebook focused on providing a deeper examination of a Splat, usually with additional rules and play gimmicks that may give an in-game edge to players who select that role for their characters. The term is a reference to the "*" wildcard character in many computing contexts, and to The World of Darkness game family (which each had its own individual block of *-books: Clanbooks, Tribebooks, etc.)
- Army books: A Splatbook for tabletop strategy games. Army books usually contain the rules and stats for the units of a particular faction, as well as in-universe history and characters, painting tips, and examples of models.
- Alternate rulebooks: Useful if the players think some aspect of the original game was underpowered, overly complicated, or a Game Breaker, or if they're just looking for something different. Or, alternatively, if the Munchkin is looking for something they can exploit. These allow the Game Master to sub in a different game mechanic for the original, while still being able to use all of the other Sourcebooks. For example, Unearthed Arcana, for Advanced Dungeons & Dragons, which introduced things like the skill point system that became central mechanics of later editions.
- Catalogs: These books are simply lists of a larger variety of spells, creatures, gadgets, or some other single category of game mechanic than the core books provide. Sometimes these are compilations of mechanics scattered between other books in smaller amounts, such as the Spell Compendium for Dungeons & Dragons.