Answer
GMS Instant Messenger allows you to maintain contact details of the people that you want to either hold IM conversations with, or whose online status (availability or presence) you wish to be informed about. These contacts are known as "buddies"
Your buddies can in turn add you to their contact details, and be likewise informed of your status.
The ability for you to see the online status of others, and for them to see your online status, is known as Presence information.
See Also:
Keywords:Buddy IM Instant Message Presence Status