You can not use yahoo messenger to talk to AOL instant messenger but you can use meebo.com to use yahoo, aol, and many other messengers all at one time. It does not have to be downloaded. Just a...
You need to have an email, if you go to the home page of your email site (Like www.hotmail.com) it will tell you to log in and then there wil be a butonto get im.Hope i helped!
Because some people like talking instantly but not on a phone or something like that.But if you want to write something super long, you might as well just use email.