Copyright’s a complex topic that comes up frequently on the boards.
In a nutshell.
Before 1978, if you wanted the government to recognize your copyright, you had to register your work with the copyright office (you still should do this if you expect to win a lawsuit). Now, though, you have recognized copyright the minute you put your work into fixed form (written or recorded).
Anything created now by an individual or group of individuals has a copyright lasts for 70 years past the creator’s death (the death of the last creator if more than one). If it’s a company who holds the copyright, protection lasts 95 years.
Before 1978, however, protection lasted only 28 years, after which you could renew for an additional term, which is now 67 years (for a total of 95). Before 1992, you had to actively renew. Now, it’s automatic for stuff still under copyright at the time that law went into effect.
Based on the timing of all these laws, the broad summary is this:
Everything created before 1923 is in the Public Domain. You may use it however you wish without permission from anyone (although you might wish to acknowledge the original creator to avoid accusations of plagiarism).
Anything registered for copyright between 1923 and 1964 is in the Public Domain unless the copyright was renewed, which it most likely was, so don’t get your hopes up.
Anything registered between 1964 and 1978 had its copyright automatically renewed for a total term of protection of 95 years.
Anything created 1978 or after is automatically copyrighted, whether registered or not for the terms I mentioned before: Death + 70 years for individuals, 95 years for others.
Copyright means you control how your creation is used and distributed. Taking an existing poem and setting it to music is called a “derivative work”, and you have the right to decide if your words are used in such a work. Same thing for taking an existing song and creating a new arrangement of it. Even if the person didn’t write the arrangement down, you still have a recognized right to control whether, and in what manner, your song is performed.