The Copyright Law of the United States provides legal protection for intellectual property. In your search for information, you should assume that all materials you find are copyrighted, unless the document specifies that it is public domain, which can be used freely by anyone. An information source does not have to be registered with the Copyright Office to be covered by copyright. It is copyrighted as soon as it is created.
The doctrine of fair use allows copyrighted works to be used for purposes such as criticism, news reporting, teaching, scholarship, or research. Fair use generally applies to nonprofit, educational purposes that do not affect the potential market for or value of the copyrighted work. Section 107 of the Copyright Law describes factors to consider in deciding when fair use applies.
The issues related to copyright of computer software, digitized images, and other products and sources are becoming more and more complicated. Some have not as of yet been adequately interpreted by the courts. Remember that all information sources and technology have been created by someone. Depending on how you use their property, you might have to ask those authors, developers, publishers, etc for permission. To be safe, do not copy anything unless you have explicit permission or a clear statement that the item is in the public domain. Whether an information source is copyrighted or in the public domain, you should cite it if you quote or paraphrase it in your paper or speech.
For further information see the Copyright Act and other important documents relating to the law and its interpretation.