Its like Yahoo IM, MS Messanger, Facebook Chat, Skype etc. i.e an IM that uses people gmail accounts or Google+ accounts & allows them to send each other fast instant messages. It is, like other IMs, are popular in the corporate world for fast inter office communications.
For example: I skype my Project Manager, who sits next to me, when he is on the phone or I quickly need to send him a link.
I saw my GTalk was down for a few minutes earlier, but it's better now. I really only use the GTalk application so the little icon can show me when I received a new email.
There actually is a vast population of gamers that use GTalk everyday (like me). I thought it was my own connectivity causing the outages. Now I can talk to my gamer friends over IM again.
Aaaaah it all makes sense now..... was getting super annoyed yesterday thought my works conenction was being kak.
EDIT: OMG was a awesome article... let me read and report back.