According the United States Copyright Office, "Copyright is a type of intellectual property that protects original works of authorship as soon as an author fixes the work in a tangible form of expression" (https://www.copyright.gov/what-is-copyright/).
By what authority is copyright given?
Clause 8 of Article 1 of the United States Constitution states:
To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries.
Copyright laws in other countries rely on similar government authority.
Copyright protects your creative work from usage by others. It gives you exclusive rights to copy, print, sell, manipulate, alter, and protect from unwanted usage by others.
What do I have to do to gain copyright?
Create an original work that is in a fixed format (recording or manuscript or other medium) that can be viewed for more than a short time. You cannot copyright material that you do not own (have not created), that belongs to someone else, or that is not in a fixed format.
Are there other exceptions to Copyright? Fair Use (and the Teach Act specifically for teachers) allow teachers and critics to use material that is copyright in order to review or teach material based on an original piece. Teachers must be in a face-to-face class or a virtual environment that is locked to only users in their course.
Do I have to apply for Copyright?
No. You need only place the original creative work in a fixed medium.
What if I don't want to protect my work from the usage by others?
You can also use a Creative Commons license that would allow others to use your work in a variety of ways, depending on the licensing that you use. Be sure to post these with your work.