Gomoku basics
What is Gomoku?
Gomoku is played by two people who take turns placing stones on a 15×15 board. Black plays first. Once a stone is placed it never moves or gets removed.
How to play
On your turn, place one stone on any empty intersection (where a horizontal and a vertical line cross). You cannot play on an intersection that already holds a stone.
Winning
The first player to line up five of their own stones in a row — horizontally, vertically, or diagonally — wins.
There's more than one ruleset
What's described above is the most permissive form (freestyle). The differences between the rulesets actually used in play — Standard, Renju, and others — are covered in the "Standard vs Renju" guide, and Renju's special forbidden-move rules are covered in the "Renju forbidden moves" guide.