Underpopulation
fewer than 2 live neighbours: the middle cell dies
Survival
2 or 3 live neighbours: the middle cell lives on
Overpopulation
more than 3 live neighbours: the middle cell dies
Reproduction
exactly 3 live neighbours: the middle cell is born
Three kinds of pattern
Still life
block: never changes
Oscillator
blinker: flips every 2
Spaceship
glider: moves 1 cell per 4