I've come across a few tutorials that make reference to
Are these classes created by Wordpress or classes I should be creating? And what are they intended for?
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Those are classes required by a particular microformat.
I personally find it annoying that they are built into the Core, meaning that if you don't want to support that microformat you have to take steps to remove it or have incomplete meta-data in the markup.
However, if you do wish to support that microformat, the page linked to will tell you everything you need to know about the available classes and their use.