Basically, a mod is something that you install that changes or modifies the gameplay. On other sites, the term can mean just about anything, but our definition is generally a little narrower here. For the purposes of Glest, a mod usually means a new faction or a tech tree. A faction is an independent groups of related units, and a tech tree is a group of factions that can play with/against each other. For example, the default tech tree in Glest is Magitech, and its factions are Magic and Tech. Technically, tilesets and scenarios could also be classified as mods, but we generally prefer to list them separately.