what do "a" and "b" mean in blender releases?

Hello.

i thought the “a” that came right after the Blender version e. g. “blender-2.75a-windows64.zip” meant Alpha, and the “b” meant Beta. I just noticed that 2.75a release came after the plain 2.75, and had about ten more megabytes of data.

What do the added letters mean and is there a log, a page, something that would clarify the changes for each?

They are updates after the release to fix critical bugs. Read the version release notes to see what was changed in each update.

http://wiki.blender.org/index.php/Dev:Ref/Release_Notes

The blender foundation uses really weird numbering conventions. Their release candidates aren´t really release candidates for one (build to be released as final unless critical bugs are found), they´re more what would be considered betas in other software (feature complete, only bugfixing allowed), and the ´a´ in the bugfix release makes you think it´s the first release in the 2.7x series (or alpha, as stated above). But anyhoo, that´s just me, and nothing that prevents anyone from using the software…

Blender release candidates are real release candidates. If no crucial bug is found in the release candidate, it becomes the release (except of a couple of changes like updating version number)