3D5 is essentially a 3D Tic-Tac-Toe game played in a 5x5x5 cube, allowing 109 possible winning combinations. It was originally a paper-and-pencil / mainframe computer game known as "Cubic." In this original incarnation, the player had five sheets, each marked with a 5x5 grid, and had to visualize the 3D stacking in his head.

One twist relative to Tic-Tac-Toe is that the center space of the cube is a "free" space that can count as occupied by either player, to contribute to five in a row. Optionally, players may specify additional free spaces beyond the center space as part of the set-up.

More recently, computer versions that allow 3D visualization and rotation of the cube have emerged. Finally, Lolo Games has come out with a physical version of the game, in which the plays are even easier to see.

This description is from boardgamegeek.com, the best source for board game information on the web.