Any recorded work of expression is automatically protected by copyright, meaning you own the rights to it. If you write a letter to your mother, it is automatically copyrighted. So is video, audio, sculptures, paintings, almost anything that is recorded. You'll often see the copyright symbol or the word "copyright" with a date and the person or organization that holds the copyright on the copyrighted work, for example on every page of a book or document. This is to avoid "innocent infringement," so that it is clear that it is copyrighted.
You can also file for a formal copyright with the government, and this gives you additional legal rights. Basically this makes it easier to sue for copyright infringement and increases the penalties. Copyright infringement is illegal. Copyrights do expire, and many things have expired copyrights, for example the Magna Carta.
Plagiarism on the other hand is not illegal, although most people consider it immoral and it will get you expelled or at least in a lot of trouble at educational institutions. Plagiarism is claiming someone else's work as your own.
Often plagiarism and copyright violation are the same. If for example you find something on the Internet and use it or large portions of it in a paper you claim you wrote, that is typically plagiarism and copyright violation. If however you hire someone to write a term paper for you, that is a work for hire and you typically own the copyright. However if you claim you wrote it yourself, that is plagiarism. Another example of something that is plagiarism but not copyright violation is if you claim something that the copyright has expired on is your own work.
Although copyright violation is against the law, there is a fair use provision. You can use small parts of copyrighted works in your works as long as you give proper credit. For example it is common to quote other people and other books or papers.
It is possible to give up or loosen the rights on copyrighted works. One example is called copyleft and another is a Creative commons license from CreativeCommons.org. For example, I wrote an ebook called "Effective Internet Presence" and wanted it to be spread as far and wide as possible, so I gave it a Creative Common license which allowed people to freely copy and distribute it as long as they left it intact, meaning they didn't modify it.
Harry writes on many topics like Intellectual Property, health, food, and wine. See his latest at best wine opener and houdini wine opener.