The basics of copyright are not so difficult to understand.
Any time someone puts a creative work in a fixed medium (such as writing a short story or book on paper, or writing out sheet music, or recording a song performance, writing a play or movie script, or performing the play or movie), that creative work is copyrighted. There are different rules for copyright depending on when the work was created, where the creator lives, and where the work is viewed/heard/etc., but in the United States at least, copyright law is mainly designed around protecting the financial rights of the copyright holder -- so to use a copyrighted work that you haven't created, you usually have to acquire some form of license (which usually costs $$$) to use that.
If you didn't create the copyrighted work, then you generally need permission from the copyright holder (which usually costs $$$) to use the copyrighted work.
There are exceptions to copyright called "fair use", but these exceptions are very tricky to determine, because ultimately, they are decided in courts of law -- fair use is a legal defense to an accusation of copyright infringement. You go to court, and then you argue to the judge that you weren't *really* infringing, because your use is a fair use. The judge decides if he agrees.
(The problem with this is that you can't know if something is actually fair use unless a judge says it is. So, unless you have the time/money/resources to risk a lawsuit, it's better to be cautious.)