If I'm not mistaken, these are for the icons. HDPI is High Definition..something or other, MDPI is Medium Definition, and Tiny MDPI is like..just a smaller version of the MDPI for display in certain things. There used to be LDPI as well, but as displays have improved, it's not used too often anymore.
Low-level devices like the MyTouch, Eris, etc use MDPI's, whereas EVO's, any Droid, etc, use the HDPI's as they have high definition displays.
I could be totally wrong, but I'm pretty it's just directly related to the definition of the icon.