Here is a short glossary for people not already familiar with the terminology. I’ll be adding to it as I think of more things that may be relevant.
PNP RPG / P&P RPG - Pen aNd Paper Role Playing Game. Broad genre of games based on the idea of creating a character, entering their “role” as an actor might, and roleplaying their interaction with the world.
PC - Player Character. The role a player inhabits, as well as all statistics related to that role. I also use this acronym to refer to players themselves. Generally it’s clear from context wherever I am referring to a character the player plays or the player themselves.
NPC - Non- Player Character. Every other character who is not a PC. If you imagine PCs as lead actors in a theatre performance played by star actors, then NPCs are everyone else - extras, secondary actors, and so on.
GM - Game Master. Person responsible for tracking rules and resolving interactions with the world, as well as roleplaying most NPCs, setting events for the PCs to face, and so on.
GMPC - A special NPC who gets allocated a very significant amount of attention from the GM. Typically travels with the party and behaves as a PC, except controlled by the GM.
Worldbuilding - process of coming up with a fictional world(making up rules for magic, designing geography, drawing political maps, designing races, creating cultures, and so on), as well as the resulting body of information about the world. Can be almost arbitrarily involved (see: Tolkien and Silmarilion, creating an entire language from scratch, etc) or very shallow.
DND - Dungeons aNd Dragons. Name of a very popular PNP RPG, sometimes conflated with the genre as a whole.
Session - Typical term for the time when all players and GM come together to play PNP RPGs. If football has “matches” then PNP RPGs have “sessions”.
Session Zero - A special session when players and GM do not play, but rather discuss all kinds of questions that have to be settled before the game begins - what everyone wants to play, when is everyone available, what rules will be used, etc.