1) Copyright covers original, tangible works. "Original" means it must have at least a small amount of creativity. "Tangible" means it must be physical. These are things that are physical: books, computer files, CDs, a list, a work of art, etc. These are things that are not: ideas, thoughts, speeches that are not recorded and don't have a script. This means many things that students and teachers commonly come across are copyrighted. For example, if you, a student, write a paper, that paper is copyrighted and anyone who wants to use it for more than "fair use" must get your permission.
2) You do not have to apply for copyright. As soon as something original and tangible is created, it's copyrighted. The work does not have to be published to be copyrighted. This means a scribble that a student does on her notes in class is copyrighted. A creator can choose to register his or her work with the Copyright Office, but it is not required in order for it to be copyrighted.
3) FAIR USE lets people use copyrighted works without asking permission of the copyright holder. Teachers and students, for example, can parody classic art works because of the Fair Use Exemption to copyright.
4) Copyright guidelines are NOT the law. They may help you stay within the law, but courts will make rulings based on the law, not guidelines (even if Congress comes up with the guidelines).
Those are just the very, very basics. There is much more to learn about the Fair Use exemption and copyright in general.
FOR FURTHER INFORMATION
Cornell Public Domain Chart: To figure out if a work is in the public domain (anyone can use it) or not.
U.S. Copyright Office Handout on Fair Use: Basic, but trustworthy information, about the fair use exemption.
U.S. Copyright Office Factsheets: Fairly dry and many of them won't be applicable, but information from the government.
Harvard's Copyright for Librarians: A huge wiki with lots of links and case studies. For librarians, but most of the information is relevant to anyone.