A site or workspace is when you want a new place for collaborating on Web pages, lists and document libraries. For example, you might create a site to manage a new team or project, collaborate on a document or prepare for a meeting.
What are the various kinds of roles the users can have?
A user can be assigned one of the following roles
* Reader - Has read-only access to the Web site.
* Contributor - Can add content to existing document libraries and lists.
* Web Designer - Can create lists and document libraries and customize pages in the Web site.
* Administrator - Has full control of the Web site.
Can more than one person use the same login?
If the users sharing that login will have the same permissions and there is no fear of them sharing a password, then yes. Otherwise, this is discouraged.