It's an image with the background cut out. They're good for sigs and wallpapers because you get a clean image with no extra bits.
AIM is Aol Instant Messenger, it's a way to chat online, not sure why you ask. PSD, or .psd, is the file type used by Adobe Photoshop. It stores things like layers and text separately, which means it's easier to edit images with it.
A render, is a single image that has the backround taken out. AIM, or AOL Instant Messanger is a instant messaging program which allows you to talk with your friends. A .psd, is a file that only Photoshop can read, as with a few other adobe products. The .psd holds far more information than a .jpeg, or .gif, allowing you to go back and change what you have made already.
Lol, you have photoshop, but don't know what a psd is That's a new one You should definitely use it, it's a great program. You can do all sorts of stuff with it.
