Superuser is the app that manages what apps on your rooted device have access to su. Apps that are granted su have elevated permissions and can modify just about any part of the system. The app was originally written by zinx, and I took it and made it much more useable. The app consists of two parts, Superuser.apk and the su binary.
Superuser.apk runs as any other app and gives you, the user, a place to see what apps you have allowed or denied, as well as view a log of what apps have used su when.
The su binary is what other apps call when they need superuser rights. The binary checks the database maintained by Superuser.apk to determine if you have already granted rights to the requesting app, and if not tells Superuser.apk to display a prompt asking you for permission.
Superuser comes pre-installed on any rooted ROM. In fact, without it, you don’t have a rooted device at all. You cannot uninstall it, it lives on the system partition with other apps that came pre-installed on your device. It can be updated from the Market if the developer of your particular ROM has used a version that is signed with the proper keys, which are publicly available on my github.
Installing Superuser isn’t somethng you would need to do under normal circumstances, but just in case here are the files needed:
ARM devices (2.0 – 4.1)
x86 devices (2.0 – 4.1)