Many games, especially those that involve roleplaying or computers, can be described with specific phrases that define their characteristics; because these phrases are long, they are often abbreviated to acronyms. Some of the more common ones are listed below.

Action
FPS - first-person shooter - This is a type of video game visualized from the perspective of the player, unlike other games where one's avatar (onscreen representation) is shown as though seen by another person, often as an overhead map view. As the description implies, a first-person shooter involves the use of ranged weaponry (weapons that shoot projectiles, like guns or crossbows; or items you can throw, like spears or rocks).

RTS - real-time strategy - This refers to a game employing strategy, where gameplay unfolds in real time, instead of by turns, as in other games; players interact simultaneously. Often such games require you to react quickly to events within the game context, to gain an advantage at an opportune moment.

Role-Playing
RPG - role-playing game - This is, in its broadest sense, pretending or make-believe as many children do informally; but in the usual context it refers to formal games with rules, where players choose a character and follow a narrative. Role-playing games may involve equipment such as dice for determining actions, and formal character descriptions. Traditional tabletop RPGs are multiplayer, but some computer RPGs, since they have an automated component, may be single-player.

LARP - live-action role-playing - This is an immersive style of RPG gameplay involving trappings like costumes and props for the characters, who act out their roles. People who enjoy re-enacting historical battles, or who perform at medieval or renaissance festivals, may be understood to be participating in LARP.

MUD - multi-user dungeon - This type of text-based adventure roleplaying game evolved on the internet before internet browsers. Early MUDs had graphics represented by clever combinations of letters, numerals, or other text symbols rather than by pixel images as they are today. The D in the acronym refers to an area where one would traditionally fight monsters in these typically fantasy-based games. Alternative words such as dimension or domain are sometimes more accurate completions of this acronym, depending on the game's theme; the shorthand MU* is sometimes used instead, where the asterisk may indicate one of several other letters or sets of letters that complete the acronym to describe various genres of MUD.

Browser-Based
BBG - browser-based game - These are games that can be played within an internet browser, without installing special software on the computer. A popular genre of browser-based game is the BBRPG (browser-based role-playing game), which can be single-player or multiplayer.

PBBG - persistent browser-based game - This type of internet game can continue over many sessions. It generally refers to some sort of BBRPG, but may describe any game where gameplay can be interrupted and continue later at the stopping point. (I've created a lens about PBBGs, and a group for others with PBBG-related lenses, here on Squidoo!)

Cards and Collectibles
TCG - trading card game - This type of game, sometimes called a CCG (collectible card game), involves special trading cards, which can be used to play a strategy game. As with regular trading cards, some cards in a particular set may be rarer than others.

CMG - collectible miniature game - In this variation of the genre, miniature figures are used instead of cards.

Online Multiplayer
When two or more people play an interactive game on the internet, it's called an MOG (multiplayer online game). When a great many people interact simultaneously, it's called an MMOG (massively multiplayer online game). An MMOG is often also an RPG (MMORPG) set in a virtual world, where all the players online at once may have a chance to interact. MO and MMO may prefix other common acronyms for game genres (for example, MMOFPS or MMORTS), or be part of the acronym for another genre that otherwise wouldn't need abbreviation (such as MMOR for online racing games).

Miscellaneous
ARG - alternate reality game - This is a sort of interactive narrative game, involving elements in multiple media, including real-world events. It is difficult to describe the nature of an ARG concisely.

PBM - play-by-mail - Chess has been played by mail for centuries. With the advent of the internet, play-by-email (PBEM) games also became popular. While some of these were the same games that had already been played in non-electronic correspondence, some games were specially designed for electronic correspondence play, and moderated with the assistance of computer programs.

WTF?
In case you were wondering, the acronym WTF in the title does not refer to a game, although it might be used in an online game with a text chat function. It is shorthand for a potentially offensive expression of confusion, which shouldn't be too difficult to figure out.