B3/S23 is the whole rulebook
Conway's Game of Life runs on a square grid where every cell is either alive or dead. Each cell checks the eight cells around it. A dead cell is born when exactly three neighbors are alive. A live cell survives with two or three live neighbors. With fewer than two it dies from isolation; with more than three it dies from crowding.
That rule is often written as B3/S23: births on 3, survival on 2 or 3. There is no score, no player turn, and no hidden random event after the starting pattern is chosen.
Everything changes at once
The easy mistake is to update cells one by one. Life does not work that way. The next generation is computed from the current generation, then all births and deaths are applied together in a single tick.
This is why the blinker works as a clean first lesson. In one tick, the vertical row creates two side births and loses its two endpoints. In the next tick, the same logic reverses the shape.
Simple does not mean tame
The rules are short, but the board is not easy to predict. The same rule set supports still lifes that never move, oscillators that repeat, spaceships that translate, methuselahs that run for thousands of generations, and guns that emit endless streams of gliders.
That is the practical reason this site treats Life as a research topic rather than a toy grid. The right question is not only what the rule says. It is what a pattern does when the rule is applied without shortcuts.
Working takeaway
When a pattern looks surprising, slow the simulator down and ask which cells had exactly three neighbors, and which live cells kept two or three. Most Life explanations start there.