Don't apologise for a 'newbie question' - we were all Newbies once
But I'm not sure if I fully understand what you are asking. If you are looking to buy from Amazon and have a Kindle then obviously the Kindle edition will work on your Kindle. But I suspect this isn't what you mean. BTW Kindle reads .mobi perfectly (although Amazon has their own format but it's essentially the same.
You mention epub. What reader do you have? If it's not a Kindle reader then you will need to convert your file if it doesn't match the format read by your reader. The very best FREE software for conversion and managing your library is Calibre (google it). This will convert anything to anything in the mobile reader range. With epub I think you are on solid ground for a several reasons. One is that there are lots of applications that will tidy up and edit epub files (eg 'Sigil', or 'mobipocket' I think) although I am myself only now learning how to edit files - but 'learning by doing' is a good way to do it.
But anyway, if you have an .epup or a .mobi or a .lit or .html etc file the Calibre will convert it for you without any effort.
If you say what reader you have then more focussed advice can be given and you might also find this link useful:
http://wiki.mobileread.com/wiki/E-book_Reader_Matrix
