The Mod vault is a feature in the FAF lobby which allows you to easily find and manage mods for Forged Alliance.
It can be accessed through the "Vaults" tab in the lobby and contains a large ammount of mods.
To find a mod, simply type in it's name into the search bar, and if it doesn't appear, press the "Server Search" button which will look through the whole mod list.
There are two types of mods; Sim mods and UI mods.
UI mods are mods which are activated on a per-person basis and have no effect on other players in the game. As the name implies, they are usually changes and enhancements to the UI and can make playing the game easier. UI mods should never make a game unrated.
Sim mods are mods which affect the game itself and are activated on a per-game basis. They range from simple mods which adjust one or two values to overhauls of the game, such as The Nomads and Black Ops. Most Sim mods make a game unrated.