What do all of the different abbreviations and acronyms (i.e., der, p7b, p12/pfx, etc.) associated with S/MIME actually mean?

The canonical place to learn about S/MIME is directly from the IETF RFCs themselves. S/MIME version 2 is in S/MIME Version 2 Message Specification and S/MIME Version 2 Certificate Handling. S/MIME version 3 is in S/MIME Version 3 Certificate Handling, S/MIME Version 3 Message Specification, and Enhanced Security Services for S/MIME.