Email messages are sent in plain text which means that the messages I send to Derpina are visible to anyone who somehow gets access to them while they are in transit.
To overcome this, various encryption mechanisms were developed: PEM and PGP are two of them.
PEM canonically converts, adds digital signature, encrypts, and sends; PGP does exactly the same.
So how do they differ? Is it that PGP (being a program) is used to generate a PEM message?
PEM was a proposed IETF standard for secure email. It depended on a single root certificate for its public key infrastructure (PKI), which was impractical and had its own problematic implications for security.
PGP started as a "proof of concept" for a less centralized "web of trust" PKI, and proved to be much more practical, finding widespread adoption and eventually founding the OpenPGP standard, while PEM faded into obscurity.
So basically PEM and PGP were competing protocols for encrypting emails, and PGP "won" while PEM "lost".
