This is a very strong chess engine, placed second in the 13th world computer chess championship 2005.
It uses the UCI (universal chess interface), for chess engines as communication protocol.
If you want to play against it, you will have to use an UCI capable interface, like scid.
This distribution comes with an opening book of moves that the engine can use for the first few moves of the game.
Using an opening book increases performance in the chess games opening.

As fruit21 uses UCI (universal chess interface) you will need a uci-compliant user interface to play against fruit21.
You can also use polyglot as an "UCI to xboard"-adapter to use togaII with xboard-compatible programs.

Fruit 2.1 was the last version released under GPL (v2); newer versions are closed source but available free of charge.

See http://www.superchessengine.com/ and http://wbec-ridderkerk.nl/ for more information.

Authors:
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Fabien Letouzey
Oliver Korff (manual page, taken from Debian upstream)
Marc Lacrosse (opening book "Performance.bin")