Horde chess is a variant where white has 36 pawns (which will be referred to as The Pawns) and black (The Pieces) needs to destroy the Horde to win. A special starting position is used:

Rules

• 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 win by capturing all the Pawns. This includes pieces promoted from the Pawns.
• The Pawns win 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 have moved two squares as it is not a valid en passant capture.

Here's a more detailed guide to horde, written by Philippe Saner.