GNU Chess is a computer program for playing chess, and is thus a computer chess program.
It is free software (in both the no-cost and as-in-freedom senses) licensed under the terms of the General Public License, and is maintained by collaborating developers. It is often used in conjunction with a GUI program such as XBoard.
In 1998-1999 GNU Chess underwent a transition to version 5. Version 5 was essentially a complete rewrite from scratch of GNU Chess to eliminate spaghetti code and replace antiquated data structures with more advanced computer chess implementation techniques. These included bitboards, a search algorithm called Principal Variation Search (PVS) (a variation by Professor Tony Marsland of alpha-beta minimax), and full end-leaf evaluation. It uses a number of other techniques to improve its performance, such as an opening book (generated by studying master games) to help it start well and hash tables storing previously-analyzed positions to prevent wasting time re-analyzing already analyzed positions.