Players
2 players.
Equipment
A game board and 12 pieces for each player (checkers or coins work well for the playing pieces).
Goal
The goal is to get your opponent down to two pieces, or to block him from making any legal moves.
Setup
The board is made up of three concentric squares, with three points marked on each side of each square (the two corners and a midpoint). The same-side midpoints of all three squares are connected by a straight line. A total of 24 points are marked on the board.
Here is a Twelve Men's Morris board (PDF).
Gameplay
Setup Phase: Players taking turns laying their pieces on vacant points.
NOTE: It is possible that the board will be completely filled at the end of the setup phase. If this happens, the game is a draw.
Regular Phase: After all 24 pieces have been laid down, the regular phase begins. In this phase, a turn consists of sliding a piece along a line to a vacant point.
Whenever a player creates a string of three pieces on one line, he immediately removes one of his opponent's pieces. (This can happen during the setup phase or the regular phase.) However, an opponent's piece which is part of a string may not be removed unless no other piece is available.
NOTE: Sliding a piece one space on one turn, then back to its original space on a subsequent turn is a legal sequence.
Winning
The first player to get his opponent down to two pieces, or to block his opponent from making any legal moves, is the winner.
