File structures in Valve games

This information is from a Windows 10 computer. Some things may not apply for users on other operating systems. It may also be different if you have a non-standard installation of your operating system, Steam, or any game.
*C:
**Program Files (x86)
***Steam
****steamapps
*****common
******Game Folders
*******mod folders
*****sourcemods
====C:====
This is your system's C drive. It probably holds everything on your computer.
====Program Files (x86)====
This is where most 32-bit programs you download will be installed to if you have 64-bit operating systems. For 32-bit operating systems, only Program Files folder exist.
<Br>All 64-bit Steam games would install on this location.
==Steam==
This folder is where most data for Steam is kept on your computer.
==steamapps==
This folder holds all or nearly all games and mods. Most things outside of this folder are just for the Steam program itself. Such as ACF files which was used to show installed games on the Steam Library. 
==common==
This holds all or nearly all the games (and some tools) you've downloaded from Steam. The folders in here are known as game folders. Some of them will have the exact same names as the game names on their Steam store pages.
==game folder==
The name of a game folder usually clearly states what's inside it; Portal 2 contains everything for the game, Portal 2. For Source games, this includes base engine things, such as Hammer and integration with Steam (for achievements, playing status, etc).
==mod folder==
Mod folders hold all the content that a regular player sees while playing a game. Mod folders can use content from other mod folders in the same game folder, using gameinfo.txt. Confusingly, one is also called a game directory sometimes.
Usually named after an abbreviation for a game, such as hl2 for Half-Life 2.
Folders under a mod folder will replace corresponding content inside a VPK. For example, if you put a file at modfolder\models\player\player.mdl, it would replace the file at VPKNAME.vpk\models\player\player.mdl.
List of all mod folders: 
===addons===
This folder is not created by default in most games, but most plugins such as SourceMod and MetaMod: Source are usually installed on this directory.
In games such as (latest Steam/Zoom version only), and all or most games since , this is where custom content was installed, such as: custom models, maps, sounds, etc...; In (or in some games), only VPK files were supported for custom content. 
This folder was also used by for it's custom content (such as Lua scripts, and other mentioned above). They either contains folders or GMA file.
==== workshop ====
Contains addons downloaded from Steam Workshop.
===cfg===
Holds many configuration files. See CFG for more info on the files inside.
===custom===
Similar to in games, except both folders and VPKs are supported. According to file included inside, using VPKs is recommended over creating a subfolders to reduce loading times.
===logos===
Contains spraypaints for a multiplayer game.
===manual===
Contains a manual explaining a multiplayer game.
===maps===
Holds all the BSP files for a game. On some games, such as , may also include LMP files used to patching the game levels without recompiling maps.
====maps\workshop====
For games that allow workshop content, maps downloaded from the workshop will be stored in here.
====maps\graphs====
Stores AIN files used by AI systems.
===mapsrc===
Usually not shipped with a game, but it's recommended you make and store VMFs in here so that your maps folder will not become cluttered. They may be also located somewhere else like (on and above).
===media===
Usually holds AVI ( or earlier), BIK ( and later) or WebM (some games) files. (Video files for things such as the startup video or FMV cutscenes)
===models===
Stores models.
===particles===
May hold manifest files (particles_manifest.txt) or PCF files.
===puzzles ===
Contains automatic screenshots and P2C files of any Portal 2 tests you've made with the in-game puzzlemaker.
===reslists/downloadlists===
See Content_Streaming.
===resource===
Contains various files dealing with text and languages, and other VGUI resources.
===SAVE===
Contains all Save Game Files.
===scenes===
Contains scenes.image.
===screenshots===
Self-expanatory. Screenshots saved in both JPG or TGA format.
===scripts===
Contains many files defining properties for weapons, vehicles, soundscapes, and more.
===sound===
Contain sound files and game music.
===.vpk===
A VPK can hold most things one would find in the folders under a mod folder directory. Some things have to specifically be placed inside or outside of a VPK.
==sourcemods==
Usually, a mod that isn't available on Steam can be manually placed here. Steam will recognize a non-Steam mod if it is in this folder and the mod is configured properly.
Category:Files