The first player puts an x somewhere on the page with lots of space around it. The second player has the first real decision: she can play an o anywhere within two squares of the x. In general, you can play anywhere that would leave all the marks within a three-by-three grid, i.e. that would leave a legal tic tac toe board. Three in a row wins, as usual.
There are more real choices than basic Tic Tac Toe, which makes this more fun, esp. if (as most of us have) you've already got Tic Tac Toe figured out.
Coming up next, Pay to Play Tic Tac Toe.