What is the "Roster" in XMPP?

I'm learning XMPP and I repeatedly see the word "Roster" but I have no idea what it is.

> contact list (in XMPP this is called a "roster")

Source: XMPP-IM - RFC 6121 § 1.3: Requirements

In XMPP, one's contact list is called a roster, which consists of any number of specific roster items, each roster item being identified by a unique JID (usually of the form contact@domain). A user's roster is stored by the user's server on the user's behalf so that the user may access roster information from any resource.

Note: There are important interactions between rosters and subscriptions; these are defined under Integration of Roster Items and Presence Subscriptions, and the reader must refer to that section for a complete understanding of roster management.

See http://xmpp.org/rfcs/rfc6121.html#intro-requirements

Let me know if anybody have any queries regarding xmpp/chat app in Android.

Roster is the name given to the list of contacts which appears in one's chat client http://www.adarshr.com/fun-with-xmpp-and-google-talk-part-2