GAIM is an instant messenger client like AOL, MSN, Yahoo and Google all have. The difference being that GAIM can incorporate all of those clients into one program. This means you can talk with your AOL buddy while chatting with a Yahoo pal.
The benefit of having all those programs combined into one is that you only need the one program running. This means your computer will run faster because system resources aren't tied up in 3-4 programs but rather in just one.
GAIM is a free software that is Open Source and free of adds. It is definitely worth a look.