The
MVV is a bit of an umbrella organization. It doesn't operate any bits of the transit system, it just ensures a consistent schedule and fare system. It's like a cooperative of a bunch of transportation companies. One ticket allows you access to all participating providers.
MVG and DB are the two primary providers. MVG is a daughter company of Stadtwerke München (which in turn is a daughter company of the city). DB is, well, DB. MVG runs the U-Bahns, Trams, and various buses. DB runs the S-Bahns. Several smaller companies run buses as well. And one MVV ticket lets you into the whole system.
The MVV webpage has some
information in English, including a list of participating providers and such.