Copyright gives a creator the exclusive right to reproduce, distribute, perform, display, and make derivatives of their work for a limited period of time. It is designed to advance knowledge and encourage creativity by rewarding creators for making or writing things.
A creator has copyright over their work as soon as they put it into tangible form, like writing it down or recording something on film.
Usually, the term of copyright is the creator’s life + 70 years. (It can be even longer than that if the work is by or for a corporation or someone’s employer.) This limit has evolved over the history of copyright in the U.S.
Anyone may reproduce, distribute, perform, display, and make derivatives of works in the public domain. Once copyright restrictions expire on a work, that work is considered to be in the public domain. Works may also enter the public domain if a copyright holder voluntarily gives up their rights to enforce copyright and declares the work to be in the public domain. In addition, U.S. Federal government (but not necessarily state government) works are in the public domain.
Fair Use is a statutory exception to exclusive rights that gives people limited rights to make certain uses of other copyrighted works without having to ask for the copyright holder’s permission to use it. For example, you can quote verbatim the words of another author in your academic essay, for the purpose of criticism or parody. Fair use designations are made on a case-by-case basis according to the four factors:
Yes. But, Wikipedia doesn’t want to be in—and doesn’t want you to be in—a situation in which there are “close calls” about whether excerpting or reusing something would be fair under the law. For this reason, Wikipedia requires you to comply with certain protocols if you think that adding that text/media would constitute fair use. Make sure your use meets their special conditions for non-free content, which detail Wikipedia’s fair use policy exceptions allowing you to post unlicensed material.
Creators may assign a Creative Commons license to their work. The creator remains the copyright holder, but authorizes specific uses of the work for particular purposes without requiring permission or payment.
When you contribute your own text or media files that you, yourself, created to Wikipedia, you automatically apply a Creative Commons license to them. For more about what licenses you're applying as a Wikipedia contributor, see: Wikipedia: Copyrights & Creative Commons Licenses: About the Licenses.
Giving credit and getting permission are not the same thing. If you use someone else’s work without credit, you may be engaging in academic dishonesty or plagiarism. If you use someone else’s copyrighted work without permission (i.e. a license) and your use exceeds fair use, this is infringement. Generally speaking, you need to both make sure you have the right to use someone’s else’s work, and also that you give credit to the creator.
Facts are not subject to copyright. It's okay to read an article or other work, reformulate the concepts in your own words, and submit that to Wikipedia. But do not just paraphrase closely; the work you submit should reflect your own original creation.