In this documentation, the following definitions are used:

QR code
Bar code
A “QR code” (abbreviation of Quick Response Code) is sort of bar code, an optical label that is designed to be machine-readable. A QR code is faster to read by computers and contains more information than than regular bar codes, which is why it is used in Monkeysign to communicate fingerprints. See Wikipedia article QR code for more information.
Message Transfer Agent
A computer program designed to transfer emails between different machines, usually running on servers. See Wikipedia article Message Transfer Agent for more information.
Mail User Agent
A computer program used to read, compose and send email, normally ran on user computers. See Wikipedia article Mail User Agent for more information.

We also try to adhere to the Modern PGP terminology when possible.