Apple Computer published a description of every ProDOS file type in use, along with other helpful documents, and included these with every distribution of the Apple II Technical Notes. They are all available here, linked from this page.
Note that, although the ProDOS file format is fully compatible with the Apple /// SOS format, and thus there are many SOS-specific file types, no descriptions are available (here, anyway) for file types used only with SOS. Also, no coverage is provided for otherwise well-documented types (TXT, BIN, SYS and so forth), and many proprietary types have no coverage either. If information on a type that you are interested in is lacking, it is because Apple did not document it here!