The board is comprised of 25 posts arranged in a 5x5 array. Players take turns placing one ball of their color on any post on the board. The winner is the first to get five balls of their color in a straight line - this can be accomplished vertically (on the same post), horizontally or diagonally.
From the instruction book -
Cinco: The newly designed game employing 25 pegs for 109 possible win positions. This fascinating game of strategy gives endless hours of pleasure and excitement.