Offering a first look at Diablo 3's upcoming expansion, Reaper of Souls, Blizzard today touched upon two new social features coming to the action RPG: Clans and Communities, both of which are designed to find and communicate with "like minded individuals."
Clans are designed to provide players with similar interests and gameplay habits a way to band together. They're a way to communicate with multiple friends at once, organize your play and just keep in touch. Clans will only have a 120 member limit as they are designed to be smaller and invitation only. Players can request to join clans, but must be accepted to technically become a member. You can also only be in one clan at any given time.
Communities share a similar focus but are intended to be much larger and casual. These are designed for players to come together for "any number of reasons," including playing the same class, visiting the same fan sites, or living in the same geographical area. "They're all about fostering relationships between like-minded people, with a keen focus on socialization," Blizzard explains.
Communities can be open to the public or private, and support "a nearly unlimited number of people." You can also be a member of multiple Communities at the same time.
Both of these features are still in development and available for testing on the Patch 2.0.1 PTR as well as the Reaper of Souls closed beta. You can read more about the specifics of each feature, including how to actually access them, over on Battle.net.