Sometimes, it's a pleasure to know that the answers to some questions are not found on the Web, because it's suggests that, after all, it's not populated so much with garbage. It's hard to explain why such questions are counter-productive and simply illogical, because explanation of some obvious things is the hardest. Please see my comment to the question.
But how to help you? Well, you may need to learn the topics you want to know, and not just these topics, all you need to learn.
In this case, please see:
Media type — Wikipedia, the free encyclopedia,
Media Types.
Don't be confused with the name "media type". It is directly related to the concept of "content type". When a header "content-type" is used, its values is a media type. Please see:
Hypertext Transfer Protocol — Wikipedia, the free encyclopedia,
List of HTTP header fields — Wikipedia, the free encyclopedia.
The same or similar headers can come in HTTP, e-mail, and so on.
Overview of managed metadata in SharePoint Server 2013.
—SA