A chess engine is a computer program that can play the game of chess. http://en.wikipedia.org/wiki/Chess_computer Any chess-playing program that you buy (or download) e.g. ChessMaster is technically a chess engine.
When people talk about CHESS ENGINES on chess.com, they are almost always talking specifically about playing THE SAME GAME of chess against a human opponent AND AGAINST A COMPUTER (chess engine) at the same time - WHICH IS CHEATING. Essentially, you feed you human opponent's move into the chess engine, then you take its reply and feed that back to your human opponent.
Can anyone explain to me what is a chess engine? Thank you.