There's really only two commonly used. It varies if you're talking videos made with some editing software like Vegas or if you're talking the big screen in theaters (minor varations) or if you're talking about TV screens and also it depends on frame size (defaults vary depending if you live in PAL land or in NTSC areas) which isn't strickly aspect ratio but rather suggested dimensions.

Isn't complicated. Aspect Ratio refers to how many units across and how many up and down. For example old style TV's have a 4/3 ratio, newer plasma and LCD are capable of displaying wide screen or typically a aspect ratio of 16/9.

The main thing to keep in mind is if you don't want you project to look distorted, don't unlock Aspect Ratio settings or you can end up with either stretched or squeezed images that can make people look fat for example. i'm fat enough without any help. <wink>

I just took a quick peek so don't know how accurate, but the following site gives quite a bit more info.

http://www.thedigitalbits.com/articles/anamorphic/aspectratios/widescreenorama.html