A modification (or mod) is an unofficial add-on for a game that adds new things, changes things already in the game, or uses the game's foundation for entirely new projects. This is greatly aided by official modding support, although especially popular games often receive mods whether they have support or not. Mods are generally treated as distinct from user-generated content because they are typically not created with tools built into the game itself, like Halo's Forge.
Especially successful modders have been known to use modding as a launchpad for games industry careers, whether at the developer of their favorite game or another company entirely. Valve in particular has made a slight habit of hiring outstanding modders and even making full-fledged games based on their work (such as in the case of Counter-Strike.) On the other hand, companies can also frown upon modding, since it can potentially involve circumventing DRM and anti-cheat systems where present, and often makes marketing official alternatives (such as microtransactions and DLC) more difficult.
Types of Mod
Rebalance / Quality of Life
One of the potentially simplest and also most involved types of mod, rebalance and Quality of Life (or QoL) mods change various parts of a game to create a more engaging and/or more accessible experience, and often mingle with New Content mods. A popular variety of these is "realism" or "survival" mods, which tweak a game's mechanics to be more punishing of mistakes, more demanding of player involvement, and sometimes more rewarding for the trouble.
However, the most common variety of these mods are simple touch-ups to in-game point values (such as item costs), fixes for bugs, or modifying aspects of a game to make them more accessible.
Retextures / Model Swaps
Replacing textures is a popular method of modding games, as textures are among the easier assets to create without professional-level tools. Some texture mods only change a specific weapon or character model, while others replace nearly every texture in the game. While some of these focus on silly or personal customizations (such as players retexturing the protagonist of Stray to match their own cat), others focus on trying to improve a game's visual fidelity (seen in "HD texture packs" for games like Morrowind), or even create completely different visual styles for a game (as is common with Minecraft texture packs.)
In more recent 3D games, especially ones without online components, modders will sometimes go the extra step of replacing game models alongside textures. This can be as simple as swapping a character's model in for a popular character from another game or as involved as replacing a model with a slightly refined one for increased fidelity with revised textures.
Rather than just modifying content already in the game, some mods add additional content not previously accessible or present within the game. This can range from new weapons, characters, and levels to entire new gameplay mechanics and campaigns, and everything in-between. Creating these mods is usually much more involved, sometimes being effectively impossible without official mod support.
These mods, alongside total conversion mods, can even sometimes involve modifying the base engine a game runs on in order to enable new features or bypass previous limitations.
These mods/games completely overhaul the pre-existing game to create a completely new experience. Some of these mods overhaul large swaths of the pre-existing game area, whereas others use the engine and gameplay present and make their own game. Some especially popular games for Total Conversions are DOOM and Quake, due to their popularity and then-cutting edge engines.
Some mods have received a lot of attention from publishers and developers, leading some mod developers to receive acknowledgement or even job offers. Additionally, some mods present ideas and tweaks which inspire future official updates for the games they were made for, or even full games.
Some of the most famous examples of modding, and especially of mods gaining recognition, Valve Software's two largest online FPS series initially began as Quake mods. The Counter-Strike series can draw its origins to the Navy Quake mod while Team Fortress originated as Quake Fortress. Interested by both mods, Valve hired their respective development teams to create retail versions of them based off of Half-Life.
A number of Minecraft's modern features have been inspired by popular mods created during the game's alpha and beta phase. Among these are pistons, which were added in direct response to the popular Piston Mod, and copper, an ore added by many mods prior to the game consolidating them into an official ore. As well, many mods extended the game's build height beyond 256 blocks prior to the build limit being officially extended in version 1.17. Creative mode is commonly thought to have been inspired by the "Too Many Items" mod, which similarly displayed and allowed spawning of any item.