What is the difference in couplers?

What is the difference between all the types of couplers on freight cars? Which are used today? Difference in Double shelf vs single shelf couplers?

Thanks!

start here: http://en.wikipedia.org/wiki/Janney_coupler

there will be a test

Typically E's and F's. Intermodal cars don't have shelfs, tankers almost always have double shelfs and every other manifest car usually has just a bottom shelf but can have none. Most typical on manifest is a SBE60(with bottom shelf).