As someone who has an online presence where I share others’ media, copyright law is something that matters to me; as a content creator it matters even more.
Far too many people remain in the dark about how copyright law works, and some are somehow surprised that yes, it does apply to things posted online. I don’t pretend to be a copyright law expert, but anyone who interacts with any kind of media in this day and age needs to at least be up to date on the basics. So here they are.
Copyright law exists to protect intellectual property — that is, original ideas and expressions. The best way to make sure you are in compliance with copyright law is to not post material that isn’t yours. Not only does sharing others’ content improperly have the potential to get you in legal trouble, it also hurts content creators and is just generally a lousy thing to do
Now, where copyright law gets murky is when the concept of fair use is introduced. This is the idea that under a few sets of circumstances, using the intellectual property of others is actually okay. For example, using intellectual property for parody is acceptable under fair use laws. (This is the concept under which Dumb Starbucks is allowed to exist.) There are some resources from Columbia University here.
The best idea here is to research the specifics of your situation since fair use and the surrounding, similar copyright law has the potential to be a bit nebulous at times.
And then you have public domain work. This is work for which either the copyright has expired, or the person who created it decided that anyone can use it. There are different kinds of what amount to public domain licenses (like the creative commons or share-alike licenses) with different sets of rules, usually involving crediting the creator of the content.
The bottom line is this: Use original work when you can, always credit content that isn’t yours, and always make sure that content you’re using that doesn’t belong to you is okay to use before you post, share, etc. So long as you treat others (and their content) with respect and don’t post things you don’t have permission to post, you’ll (probably) be fine.