Copyright is a legal term that means that someone has the right to copy something. It could be a book, it could be a photograph, it could a drawing, it could be a movie, it could be just about anything. Copyright protects creators from having their works stolen and distributed for free.
If you're confused about copyright, it's best to play it safe and not copy stuff. If you're in business, it's wise to know a good copyright lawyer. I know a real good one, who came to my Graphic Design classes every semester and did a talk about copyright. As a teacher, I learned a lot, mostly that in spite of being a career Graphic Designer, I really didn't know much about copyright.
One of the most common things I still hear nowadays is that it's OK to post copyrighted stuff as long as you give credit. No, it's not. No. No. It's a courtesy to give credit to a creator who has given permission, or sold the rights for you to use his copyrighted material, but credit isn't enough. If that were true, all I would have to do would to credit Steven Spielberg and I could post Jurassic Park right here. Of course, I can't, Steve and the Netflix people would be after me right away. So, no, just giving someone credit isn't enough to give you the right to post their copyrighted material.
Copyright here in the United States began with the establishment of an empty building in Washington, D.C. called the Library of Congress right after the U.S became a country. And ultimately, everything that starts out as copyrighted loses its copyright, and goes into what is known as public domain. When the laws were first written, when people didn't live as long, it was about long enough to protect the creator and their children. After a few years of people living longer, the time was extended. And the point here is that the copyright expires, and whatever was created goes into the library, which has been pretty full for quite a while now. But you can still add more things, by going to the LOC web page.
Of course, you don't have to file for copyright, and you haven't really needed to, legally, since 1977, when the law changed to protect a creation "from the instant it was created". So, everything you draw, every photo you take, is protected by copyright. But waitaminute, it all comes down to enforcement. If you're thinking that someone might want to steal your copyrighted creation, such as a movie, it's worth the extra effort to file a copyright notice with the Library of Congress. Copyright lawyers call that your "ticket to court". Without it, it could be proven, but could get very expensive (lawyers charge a lot per hour!).
I hope this helps. And I hope that you can see that everything that everyone does, even their photo of their dog, is copyrighted. If you re-post that picture that someone took, yes, you're in violation of copyright unless you get their permission. When I post photos that aren't in the public domain, I always get permission. Of course, if someone has a really great photo, they'll grant permission only if they get money. Fair is fair. And it doesn't matter if they've traveled to the top of the Himalayas to take that photo or if it was in their kitchen - if it's their photo, it's their copyright. If you use it without their permission, even if you give credit, you're stealing.
By the way, if you like this blog post, please feel free to use it, to share it, to copy it. I'm giving away my copyright to this blog post. I can't imagine that it would be worth much, anyway! Oh yeah, and you can use my cartoon, too. You don't need to ask my permission, or pay me anything. I would be flattered.