When the first major Diablo 3 expansion releases on March 25th, there will be a major patch that adds new social features to Diablo 3. Diablo 3 v2.9.1 will add clans and communities to the main game.
Clans will be limited to 120 members and are founded on an invite-only basis. You can also only be part of one clan at a time.
Communities is a more casual association that can be set to public or private. Unlike Clans, membership is expected to be ‘nearly unlimited’ and you can be a part of multiple Communities at one time.
The update is expected sometime before the launch of Reaper of Souls and is currently available to test out on PTR.