OK Tinks, in english
You currently use MSN Messenger. The program on your PC connects to a server program out on the big bad internet. You friends also connect to this server(or another one that is also connected to the one you use). When you send a message it travels to the server and is passed up to your friend. MSN is a set of servers owned and run by Microsoft. The program you use talks in a certain 'language' . Other people also have written a similar systems, AOL do one so do Yahoo. These are the big names on the internet. There is also a smaller system called Jabber. If MSN YAhoo and AOL start charging you just need to move to using Jabber instead. It will always be free. The only issue is that each system has its own 'language' hence MSN cannot talk directly to AOL or Jabber. You need to be on the same system to talk to your friends. GAIM is a program that understands all these languages so you don't need lots of programs to talk to all your friends on all these different systems.
The bit about my work is that I run a setup just like MSN only on a very small scale. It isn't available to people outside my team at work(albeit 4 of them are in the philipines, 4 are in Knutsford, 1 is in Poole). If I wanted to(and work allowed) I could tell it where to look for an Internet based Jabber system and I would then be able to talk to anyone in the world who uses Jabber.
The last bit is a website that gives out all the latest gossip to do with computers and large companies computer systems.
Hope that helps!