Stasi is the abbreviation for Staatssicherheitsdienst. In the GDR this referred to the Ministerium für Staatssicherheit.
The Stasi monitored the people of the GDR, especially those with contact to people in West Germany. Letters were opened, read, and re-sealed. Telephone calls were recorded.
Many people co-operated unofficially with the Stasi, the so-called “IMs” (inoffizielle Mitarbeiter) and reported information about their friends and neighbours. The Stasi then archived this information, creating a large source of information about the people that they were monitoring.
Much of this information was destroyed before German re-unification, although there are projects that reconstruct these documents leading to new revelations about who informed on whom.
To hear a simple explanation and a short discussion in German, listen to the podcast:
(Press the “play” button to listen to the podcast)