Video Games Encyclopedia

Many types of MMO games can be classified as casual, because they are designed to appeal to all computer users (as opposed to subgroup of frequent game buyers), or to fans of another game genre (such as collectible card games). Such games are easy to learn and require a smaller time commitment than other game types.
MMOPGs, or massively multiplayer online puzzle games, are based entirely on puzzle elements. They are usually set in a world where the players can access the puzzles around the world. Most games that are MMOPGs are hybrids with other genres.
There are also massively multiplayer collectible card games. Other MMOCCGs might exist but are not as well known.
Alternate reality games (ARGs) can be massively multiplayer, allowing thousands of players worldwide to co-operate in puzzle trails and mystery solving. ARGs take place in a unique mixture of online and real-world play that usually does not involve a persistent world, and are not necessarily multiplayer, making them different from MMOGs.