If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
In this game you're an alien who tries to destroy everything that the human beings have built on planet earth. You have several weapons at your disposal: the more points you earn, the bigger the choice of weapons. The green meter indicates how much ammo you have left: if the level's too low, it's temporarily impossible to use your weapon. If you click on the green button, a map will appear, with all the areas on it that you can choose. Click on the blue 'Achievements' button to see which goals you have to achieve.