This classic is about finding the same two pictures. You can turn the cards two at a time. When you've found two of the same pictures the cards stay facing up. Otherwise, they flip back over again.

The idea of this game goes back to 17th Century Japan. There it was known as Kai-awase. Both then and today, the concentration game to train your memory has been a worldwide hit. There are even tournaments every year.
With gimigames you can now play it as Money Memo.