Under U.S. law, copyright protects creative works that are in tangible form: books, pictures, plays, movies, musical compositions, sound recordings, and architectural works.
What's not protected: ideas, titles, and facts, such as data. Works created by the federal government are also not protected by copyright.
Copyright owners have six specific rights under the law:
Although you might be familiar with the ©, you do not need to include it anymore to copyright your work. All creative works in the United States are now protected by copyright automatically as soon as they are created, although this was not always the case.
Generally, authors automatically own the copyright to their work. Students are also the owner of the copyright in their own works, including their assignments.
Because the point of copyright is to ensure the public benefits from scientific and creative advances (this is actually stated in the U.S. Constitution), copyright does not last forever. Once a copyright term has ended on an item, that item enters the public domain and anyone can use it however they want without having to ask for permission.
How long copyright lasts has changed over time, though, meaning it's not the same for all copyrighted works. Most anything made in the past few decades is likely copyrighted for the life of the author plus 70 years. Anything published before 1923 is now in the public domain. Please see this Copyright Term and the Public Domain chart from Cornell University to help determine if a work is still protected.
Many people will often conflate plagiarism and copyright, arguing that not giving someone credit infringes the author's copyright. Some people also think that as long as they give an author credit, then they are not infringing the author's copyright.
However, plagiarism has little to do with copyright. Except for a small amount of cases involving artistic works that are meant for public display, U.S. copyright law provides no protections for giving authors credit. This means that in most cases, while plagiarism is unethical and seen as academic misconduct, it's not actually illegal.
U.S. law also does not provide any protection to users just for giving credit to authors. Students and faculty should always make sure they're giving proper credit but should not rely on that alone for protection against infringement. Instead, make sure your use meets the requirements for fair use.