If you wrote an essay or article, you are the owner of that article unless and until you contract away your rights (such as in a publishing agreement).
Giving away the bundle of rights that constitute copyright is often called a grant. If the transfer is exclusive it has to be in writing. In books/articles, this usually occurs in a publishing agreement.
The Creative Commons has developed a series of licenses that allows copyright holders to retain control over their works, but still make them available under terms more favorable than copyright allows. Essentially, under the creative commons licenses, owners of copyright have allowed others to use their work with certain limitations specified in the creative commons license.
Q: Should I put some sort of copyright notice on my work?
A: It is wise to do so because even though it is not required, many people misunderstand basic copyright law rules. So, putting a notice on your work will remind others not to use it unless they have an exception applies to general copyright rules or they have obtained your permission first.
Q: As long as something is for educational use, I'm not violating copyright laws, right?
A: Unfortunately, no. Although there is a limited exception for face-to-face teaching, not all educational uses of copyrighted works will fall under that exception and fair use is decided on a case-by-case (not a blanket exception) basis.
Q: How do I know FOR SURE that something is a fair use?
A: That's a tough one. Unfortunately, it is hard to know when something is a fair use for sure because, ultimately, the court decides fair use cases on a case-by-case basis. Generally, we should exercise our good faith judgment and consider risk assessment when making fair use determinations. But, this does not mean that we shouldn't exercise our fair use rights. We should do so in a considered way.
Q: What role does licensing play in specific copyright questions?
A: A very large role. Essentially, you can contract away (through licensing) any of your copyright rights. So, for instance, if I write a journal article but if I assign my copyright entirely to the journal publisher, then I no longer have any right to share my article either publicly or privately without the permission of the journal.