Telemann was very prolific. That means that he wrote A LOT. In fact, he wrote more pieces of music than almost any other composer in history - more than 3,000 pieces! He was also almost completely self-taught.
Telemann knew Handel, and was close friends with Bach. Today, Bach and Handel are the more famous German composers of the baroque period, but back then, Telemann was actually much more famous and well-respected than either Bach or Handel.