The key elements of online communities are (and I know these are open to debate):
Asynchronous interactions – learning conversations take place over a period of time and anytime (they are not time dependent).
They are not location based – wherever you have access to a device connected to a network you can access the community – e.g. using my Smartphone on the 6.05 am train into Glasgow.
They are – and I know this has been contended – non-hierarchical – online interaction has none of the social clues that are a key element of and influence interaction in the physical, face-to-face world.
Social collaboration is the focus for learning – its learning through conversation.
And very often the learning outcomes can be unintentional as learning is co-constructed by participants.