The Copyright Clearance Center has loads of information to help with copyright questions, including this 6-minute video that breaks down the basics. The Copyright Alliance is another organization with a very helpful website, including a section specifically for Educators.
Ever wonder if that manuscript, book, image, etc. is protected under U.S. copyright laws? The American Library Association has come up with a fantastic tool to help answer that question: http://librarycopyright.net/digitalslider/. It lets users scroll through numerous copyright terms for works published or created during different time periods and under various conditions.