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?