Hi Joe,
An OU is basically a group in active directory. Could be PC's, could be Users. You can create an OU for example for Laptops and one for Desktops to help you organise the way you look at them.
Furthermore, you can link specific Group Policy Objects (GPOs) to different OUs. For example you can enable Desktop Redirection for Desktop machines only, and not for laptops.
Hope it helps a bit.
Chris