Bassically it's just a way to chat between servers, you dont have to join any, but if you have friends on Union etc you can create a Global Channel and talk to them via that channel, its a realy usefull feature, but it's bugged at the moment so it drops every so often when you Zone in and out, but don't let that put you off it. It also uses your global handle, so when you chat it isin't said by your character name it's said on your global name, your global handle is usualy your first ever character name but you can change it only ONCE so becareful when you change it.
Hope that helps and welcome back to the game ! 
Thanks very much for the reply, I keep seeing people talking about a forum channel, how would I go about joining this?
It is good to be back, the game just seems to get better everytime I come back.
Hey guys and girls, whats this new global chat thing all about, i've come back after a 6 month break so i'm not up to speed with it.
Do I have to join a specific channel?
Cheers