An ActivityStream is a feed format, like RSS but with more structure. The Activity Feed is the regularly updated part of a profile that shows what a community member has been up to.
2) Possible feed sources would be Bugzilla, WikiMO, AMO, etc.
3) User's ActivityStream Sources are regularly checked for updates.
5) The profile's list of activities are in reverse chronological order and give a sense of how active a user has been lately.