The roster lets you keep track of the availability ("presence") of other users. A roster also allows you to organize users into groups such as "Friends" and "Co-workers". Other IM systems refer to the roster as the buddy list, contact list, etc.
Every user in a roster is represented by a RosterEntry, which consists of:
• An XMPP address (e.g. email@example.com).
• A name you've assigned to the user (e.g. "Joe").
• The list of groups in the roster that the entry belongs to. If the roster entry belongs to no groups, it's called an "unfiled entry".