An Application (or App) is the main representation of a product on Steam. An App generally has its own store page, it's own Community Hub, and is what appears in customers' libraries. Each App is represented by a unique ID called an App ID - that you'll see referenced throughout this documentation and used with the Steamworks API and Steamworks Web API. Generally a single product will not span multiple Applications. (source)

Some games have achievements that are no longer reasonably or actually attainable. While SAM can be used to abuse the achievement system, it also lets people who do care about achievements unlock achievements that would otherwise be impossible. One common example is achievements requiring you to play multiplayer on a game that no longer has any active players, or even dedicated servers. SAM is a potential solution for a game's poorly designed achievements.

By default, SAM stores your user settings in %LOCALAPPDATA%\SAM (C:\Users\<username>\AppData\Local\SAM). Inside are additional directories and/or files depnding on what you has been saved, cached, etc.