Superuser is an application for granting and manage Superuser rights (su) for your Android phone. Apps that are granted su have elevated permissions and can modify just about any part of the system. Superuser 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.
And also a duplicate of SuperSu, to be more specific, SuperSu it's just an update for that SuperUser aplication, however, both apps do pretty much the same thing, so as SuperUser was dicontinued they came up with SuperSU.