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The MOBI file format has an internal file format version (FFV) number. Versions up to 7 are all based on HTML 3. Higher version numbers indicate that the file includes additional features that were not present in the original Mobipocket e-book format. However since all Kindles support at least up to version 7 the difference between that and lower versions is usually unimportant. Amazon just calls this format MOBI or MOBI7.
File format version 8 (also known as Kindle Format 8 or KF8) was a major change to MOBI, based on HTML 5.
A Master-MOBI file contains two formats in a single file, a MOBI7 part and a KF8 part. It also contains mapping of equivalent content between to two formats, page number information, and higher resolution images.
The newest Kindle format 10 (aka KFX) has a totally different structure and is only loosely based on HTML.
The names beginning with "azw" refer to file extensions used by various Kindle apps and devices. Amazon is not consistent with file extensions across apps and devices so there is not a one-to-one mapping between formats and file extensions.
".azw" was originally used for MOBI7. However some Kindle apps use ".azw" for any Kindle format.
".azw1" is used for Topaz, another proprietary Kindle format.
".azw3" is used for KF8.
".azw4" is used for a MOBI file that contains an embedded PDF document instead of HTML content.
".azw6" is used for HD image container associated with a book in KF8 format.
".azw8", ".azw9", and ".kfx" are all used for KFX.
".mobi" sometimes refers to MOBI7 and sometimes to a Master-MOBI.
".prc" refers to either MOBI7 or KF8.
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