Does anyone know of a source that describes all of the different types of switchgear protective relays (i.e. 50/51) with the numbers used and a description of what the relay does?
ieee.org They are the oficial source for device number designations.
Below is a link to a quick reference list, but for more detailed descriptions you should really invest in buying the standard paper from IEEE. You can also do a Google search for "ieee device numbers" in quotes just like that. You will get hits on lots of other people's interpretations of the IEEE standards, but only the official one counts.