Original: Marzahn-Hellersdorf is the tenth borough of Berlin, formed in 2001 by merging the former boroughs of Marzahn and Hellersdorf.
Simplified: Marzahn-Hellersdorf is one of the 12 new boroughs of Berlin. It was created in 2001 when the old boroughs of Marzahn and Hellersdorf were merged (joined together).