copyright is a bloated and terrible mess of legal bullshit. it sucks for just about everyone except massive IP-hoarding megacorporations, and is a net negative for society as a whole. despite all that, you still ought to understand the basics of abiding by it if you don't feel like getting hit by a DMCA takedown.

hey! note that this article is mainly focusing on USA copyright law. also, i'm not a lawyer; this is not legal advice.

first of all, copyright applies to just about everything. if a person was involved in making it, and it has any amount of creativity involved in it, that work is going to be protected under copyright by default. while this may sound like basically everything — and it is, honestly — there are some notable exceptions, such as:

• works created solely by non-persons, whether they be animal or computer
• works lacking in originality, such as reproductions of existing works, or instructions

in general, you should assume that unless otherwise specified, you do not have permission to use something. even if it was shared on the internet freely for you to look at, that does not give you permission to share it, edit it, or try to claim it as your own.

however, that is not to say that you can't use anything you find online like that. in fact, there are plenty of works available under terms that allow you to do exactly that!

the most obvious example is public domain. generally, anything old enough automatically enters the public domain. such works can be used freely for any purpose, without any sort of restriction; so feel free to use those old books, songs, paintings, and photographs as much as you want. some works are also released under a license that is functionally equivalent to being in the public domain — referred to as a "public domain dedication" — and can therefore be used in the same way.

Creative Commons licenses can provide similar freedoms, but you must pay attention to the terms of them. the most common is "attribution"; that is, to provide credit to the original author if creating reproductions or derivative works. "share alike" requires any derivative works to be licensed under the same license as the original. "non-commercial" prevents commercial use — whatever that means — and "no derivatives" means you can't make derivative works at all, only reproductions.

for other licenses, you'll just have to try and make sense of them yourself. if you're not sure if you can use something, don't use it; you're better off safe than sorry.