Sure it'll convert Kindle books (but you'll need to use the Calibre plug-ins to remove the DRM, if the book has DRM. We can't talk about that here, but a Google search for "Apprentice Alf" will get you everything that you need).
Calibre does show you the book's format. You can store the same book in multiple formats. Kindle books will either have a format of "Mobi" (for the old "Mobipocket" format), or "AZW3" (for the much better, more modern, "KF8" format).