This is called Tower Defense. As the name suggests, you defend towers. Click on the arrow to the right to see the possible purchase options. Click on a tower and drag it around the screen. Click on the arrow button on the top left to start the wave. Below it is your money and at the top is how much lives you have left. The orange enemies are normal, yellow is fast but less health, and red is strong but slow. Click those grey heart and star to make them yellow and red.
Thanks to @warfame.
