1. A cellular-automata game invented by John Horton Conway and first introduced publicly by Martin Gardner (Scientific American, October 1970); the game's popularity had to wait a few years for computers on which it could reasonably be played, as it's no fun to simulate the cells by hand. Many hackers pass through a stage of fascination with it, and hackers at various places contributed heavily to the mathematical analysis of this game (most notably Bill Gosper at MIT, who even implemented life in TECO!). When a hacker mentions ‘life’, he is much more likely to mean this game than the magazine, the breakfast cereal, or the human state of existence. Many web resources are available starting from the Open Directory page of Life. The Life Lexicon is a good indicator of what makes the game so fascinating.
2. The opposite of Usenet. As in “Get a life!”