Online communities are groups of people brought together through a common site or common community who are communicating mainly through the Internet.
They may simply have a shared interest to talk about ... or more formally learn from each other and find solutions as a Community of Practice.
Online communities may use email lists or forums, where content is centralised. Communities may also emerge from conversations around or between bloggers.
List or forum-based communities can be difficult to join up with blog-based communities because of the different ways they operate technically.
While communities do emerge organically, some community-building is necessary if there are specific goals to achieve.