Calibre is a multi-function ebook management tool. In terms of file types, however, it serves a special purpose in allowing users to convert ebooks from one type to another.
This includes the ability to convert (non-DRMed) Amazon books into other formats and vice-versa so they can be read on devices other than Kindles. A complete list of file types that can be converted using Calibre is available.
Note: Calibre itself can't covert files of books with DRM. However, there are Calibre plug-ins that do have that capacity and can break DRM. This is a violation of federal law.
Open Standard File Types
Closed Standard File Types
Digital Rights Management File Types