There are several major IM services, where basically each allows you different things, though essentially they all allow you to chat with others.
I've used several IM's in the last few years, and think it's really a matter of preference. Not always your preference, by the way, but those of your friends.
But you don't have to decide. You can sign up with any that you like, and see which is most suited to your needs. It's free.