Avatar: refers to a player-controlled character in a game.
FPS: stands for "first-person shooter," a videogame genre characterized by action-oriented gameplay featuring a first-person perspective camera and the use of firearms.
Live-streaming: similar to live-broadcasting, but audio/video feeds are transmitted via the Internet. An increasingly common method of video production.
Mod: short for "modification" or "module" (depending on who you ask), mods are digital files that are executed in-conjunction with a pre-existing piece of videogame software. When executed, mods alter the software in ways that range from significant to imperceptible.
Modder: a person who creates mods; typically, modders are characterized as amateur users, not professionals (i.e. people employed by the gaming industry).
Modding: the practice of creating mods.
NPC: stands for "non-player character," referring to interactive characters in games other than player-controlled avatars.
ROM: colloquial term for a digital copy of a complete videogame. Stands for "read-only memory," referring to a type of data storage designed not to be copied.
RTS: stands for "real-time strategy," a videogame genre characterized by tactical gameplay that unfolds over time without frequent pausing, usually featuring a top-down, panning camera.
Total conversion: a category of mod that uses a pre-existing videogame as a platform for creating a new game that is different from the original on nearly every level; still requires the original game to function.