Simple. .mobi and .azw are exactly the same thing, just slightly renamed.
MOBI is a legacy format acquired by Amazon from Mobipocket. Lots of books in that format, and it's the default way of showing them.
AZW is any MOBI book that passes through Amazon's servers, renamed to reflect that fact. Mostly because any book that comes from Amazon's servers is *probably* purchased from them, and has a high likelihood of having DRM.
It gets even more confusing since it can also be PRC, which was used on Palm readers, since apparently the Palm OS cannot sense any filenames that don't end in .prc (Palm Resource Code) for no good reason. So mobipocket books had to be renamed, and some of them are still in this mythical "PRC" format.
To be clear: The actual ebook is is identical, byte for byte. (Except for any metadata changes that may have been made, so maybe it's not so clear.
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