The simplest definition of copyright is a property right given to authors that allows them to control, protect, and exploit their artistic works.
Today it is very easy to receive a copyright for your work. All it needs is to be original, a little creative, and fixed in a tangible medium of expression. So, as soon as you create something, it is probably copyrighted. You don't need to do anything else; you don't need to file paperwork with the federal government, pay any fees, publish, or do anything other than create.
The statutory basis of copyright is in Title 17 of the United States Code. However, be aware that the interpretation of various provisions of copyright law may differ depending on the judicial circuit.
The United States Copyright Office offers a number of guides for a general audience, called "circulars," about copyright law.
There are three basic elements that a work must possess in order to be protected by copyright in the US:
Originality: To get a copyright, a work must be the original work of the author. This doesn't mean it has to be "novel" in the way an invention must be to get a patent. Instead, it basically means the work can't be a copy of something else.
Creativity: The U.S. Supreme Court has said works only need to have a "modicum" of creativity to be creative enough for copyright.
Fixation: For works to have copyrights, they cannot be purely ephemeral. You can't get a copyright just by speaking out loud. But, as soon as you record that speech in some way, then its "fixed" and you may have a copyright over it. Notably, courts have interpreted this to mean that something as temporary as being copied onto RAM memory can constitute fixation.
The bar to satisfy these three elements is very low and most works will easily do so.
Rather than being one right, it might be best to think of copyright as a bundle of rights which authors have over their artistic works. Those rights are:
As a creator, you can give your whole copyright away, or you can allow others to use some or all of these rights through licenses. The choice is yours.
The law creates eight categories of works that copyright protects. These are:
Copyright does not protect:
This means, for example, that some charts, table, and graphs are not actually protected by copyright. For more information, see Copyrightability of Charts, Tables, and Graphs.
In addition, copyright is not available to works of the US government (works "prepared by an officer or employee" of the federal government "as part of that person's official duties").
The copyright status of works created by US states and territories is less clear: see the State Copyright Resource Center.
The default rule is that authors own the copyrights to their works as soon as they create them. Like so much in the law, however, there are several instances where ownership is not so simple. Joint authors, for instance, both own the whole work together, instead of each person owning the part they create. What is more, if employees create works that are within the scope of their jobs, the copyrights are owned by the employers as "works-for-hire."
Additionally, the law permits copyright owners to transfer their copyrights in whole or in part to other people, as well as allow others to use their works while retaining copyrights ownership through licenses.
Furthermore, the law specifically allows people to transfer their copyrights in part or in whole. Note: a transfer of copyright ownership is not valid unless it is in writing and signed by the owner.
Notably, copyright protects the expression of an idea, but not the idea itself. This should be intuitive: Copyright is designed to protect art not the ideas art is based on. So, just because copyright protects the Harry Potter stories does not prevent another person from writing a completely unique story about a boy wizard using magic to fight evil. The law protect's J.K. Rowling's specific telling of that story, but not all of the ideas that are contained within the story.
Importantly, there is a distinction between plagiarism and copyright. Plagiarism is the failure to give proper attribution to an original creator's work when another person uses the original creator's thoughts, words, or ideas in a secondary work. Whereas, copyright infringement is using some or all of a copyrighted work without the original creator's permission, or without an exception that allows one to use such a work.
While copyright infringement may overlap with plagiarism, the two concepts do not serve the same purposes. Indeed, simply providing attribution for a work's creator does not protect you from violating the author's copyright. Similarly, just because copyright law might allow you to make certain uses of another person's work does not mean that such a use would not be an instance of plagiarism.
So how do you avoid them both? The simplest solution is this: Always provide attribution for the source of a work and consider whether you have a right to use a work under copyright law. Both are important and one will not substitute for the other.
For more on the difference between copyright and plagiarism, read this post from Plagiarism Today.