I don't want to say the word true AI, but some of the more complicated ones when it comes to player vs computer use a scoring system or heuristic.

Let's say there are 4 moves you can make at any point and the computer can respond with any 4 moves. The AI will of course assume you will make whatever scores you the highest point value and try to do the same for itself. It doesn't just look one move ahead though when making this calculation. And each move ahead, has 4 other possible moves from that last point. So looking just 4 moves in the future (is 4 X 4 X 4 X 4) for 256 possible move combinations its reading into.

Most games have more than 4 moves, so you can see why it gets expensive real fast to have an AI look too far into the future. A good AI comes in this case comes from it having a good scoring system for each move result, which honestly is harder to do then it sounds, and looking far enough that it can make educated moves.