The roster buffer is called *-jabber-*. It simply contains a
list of the contacts on your roster.
In the roster buffer, any command which requires a JID will default to the JID under point when called. These commands can be called through either keyboard menus or mouse menus. To open a menu with the mouse, simply press the second mouse button over the JID in question.1 This will bring up a menu with all available actions. The keyboard menus are split into categories: Chat, Roster, Information, MUC (Multi-User Chat, or groupchat) and Services, opened by C-c C-c, C-c C-r, C-c C-i, C-c C-m and C-c C-s, respectively.
You can call jabber-display-roster to redisplay your roster
according to changed preferences (see Customizing the roster buffer). This will not refetch your roster from the server.
Refetching the roster is usually not needed, since updates are pushed
to clients automatically.
You can choose not to have the roster updated automatically on
presence changes (see Presence alerts). In that case, you need to
call jabber-display-roster manually.