Destroy the horde to win!
Horde chess is a variant where the white side has a lot of Pawns (36 to be exact) (therefore shortnamed "the Pawns") and the black side ("The Pieces") needs to destroy the Horde to win. A special starting position is used:
All pieces moves similar to standard chess. In other words, a move is legal if and only if it is legal in standard chess for a similar position. There is an exception for the Pawns.
The Pieces wins by capturing all the Pawns. This includes pieces promoted from the Pawns.
The Pawns wins by checkmating the King of the Pieces. Pawns on the first rank may move two squares, similar to Pawns on the second rank. However, Pawns of the Pieces may not capture Pawns on the first rank that has moved two squares, as it is not a valid en passant capture.
Here's a more detailed guide to horde, written by Philippe Saner.