Community (Site) vs. Organization

Community	Organization
The users of the portal do not fit in any hierarchy. They are like an ad-hoc group of people and join or leave the group at any point of time.	The users of the portal fit in some sort of hierarchy like the Company-Department-Team
There is some function or area of interest which cuts across the entire structure of the portal.	When a user and the group has very tight coupling, for example, the user cannot join or leave the group at his/her will very easily. When you have some kind of delegation chain in the hierarchy. When you want the admin of the group to manage the member's profile.