Aol Instant Messanger, or AIM, as it is commonly refered to is a stand alone program that allows you to chat with others who have AIM (or similar programs) regardless if you have AOL internet service or not. It's just like MSN messenger, Yahoo messenger, or ICQ. As long as you have an internet connection, no matter what type or from whomever, you can connect and chat with other people.

However, AOL Instant Messenger will dump crap onto your computer that you will most likely not want, such as ads for AOL Broadband. AOL instant messenger now has ads on it that have audio (very annoying). Though, AIM is still the most popular with most people.

MSN Messenger has bugs and flaws that pose potential risks towards your computer. Just remember to keep your Micrsoft Windows up to date and apply available patches as soon as they come out.

Yahoo Messenger has flaws and holes that easily allow others to boot others from chatrooms and/or Yahoo Messenger completely. However, many people consider Yahoo Messenger most user friendly.

As far as ICQ goes, I have heard good things about it, but I have not used it first hand.