What is Copyright?
According to the U.S. Copyright Office, copyright protects "original" works when the owner "fixes" that work in some form of semi-permanent publication. The main requirement for something to qualify as copyright is the creative aspect. It must be "minimally creative," which is defined as including some aspect of creativity or originality. This makes copyright applicable to almost every type of published creative works. However, there are some kinds of works that are not protected, such as names, titles, short phrases, and small variations on already created works. Unlike trademarks and patents, you do not have to apply for copyright. As soon as you publish your creative work, it is protected under copyright.
Keep in mind that copyright can not, by definition, protect "ideas." Copyright protects the versions of those ideas that have been fixed. The creative expression of those ideas is what is protected.
Although it is not required, you can register your work for copyright, which will not only strengthen your claim as a copyright holder but will also allow you certain privileges that standard copyright does not. One of the major benefits of copyright registration is in the event of infringement. The registration will benefit you if you, or the company holding the copyright, want to pursue litigation. It may even allow the holders to claim financial damages when in court.
Why is Copyright Relevant to You?
As mentioned before, you do not have to apply to obtain copyright over the work that you publish. And because "publish" extends to any form of sharing ideas in a semi-permanent manner, you (yes, you) are a copyright holder. Every post online is protected under copyright. This also means that you are held responsible for your copyright infringement. And though there are forms of "fair use" (that will be discussed under the "Fair Use" page), without the proper knowledge, it is easy to accidentally infringe on someone else's copyright.
Copyright might not seem like the kind of thing that is relevant to you right now. You might be thinking, "Who cares about protecting the memes I post on Instagram?" However, even the most frivilous of creative works are important to the open discourse and exchange of ideas that create the kind of environment that encourages innovation. You may not care about protecting your 4:00 AM joke tweets, but eventually, there will be a creative work that deserves the kind of protection copyright offers.