Title 17 of United States Code defines and details Copyright Law. (For the latest and complete version of the U.S. Copyright Law, see this page.) The law is designed to protect "intellectual property." It protects the person or entity who creates a work from others claiming the work as their own or making a profit from it. The original work being protected can be print or electronic in its form. It can be words, software, music or art. A person doesn't have to register their original work with the U.S. Copyright Office for the work to be copyrighted. Original work is protected as soon as it is created by the copyright laws of the U.S.
You should always assume that any material or resource (in any format) that you find is copyrighted and therefore protected by law. Only those works specifically identified a being in the "public domain" can be reused without permission. Still, it is proper academic practice to adequately cite even information from public domain sources like a 19th century novel or a government website when you are quoting from them in a class presentation, school project or research paper.
Here is a small excerpt from a U.S. Copyright Officer Circular:
To see the entire 12 page pamphlet, click here. Their FAQ on Copyright can be found here. And see the entire Copyright Office webpage here.
For a basic introduction to Trademarks (a protected "word, name, symbol, device, or any combination, used or intended to be used to identify and distinguish the goods/services of one seller or provider from those of others, and to indicate the source of the goods/services") first visit this government website for The U.S. Patent and Trademark Office. You can get information on Patents ("an intellectual property right granted by the Government of the United States of America to an inventor") at the same website. Also look for books here and at the Law Library. Here's an easy-to-read example: