I really love this game and one of the reasons is power-ups. I always enjoyed games which offered various different features, different options and functionality.
So in the game you can grab different weapons, only one type of the weapon can be activated at a time. These weapons may include guided missiles, more ammo, bigger shells, tracers, lasers, etc. They appear as a small grey icons all over the map, you need some luck in order to find best power-up (in my case I enjoy to use guided missile the most.)
Pay attention to the bot right corner of the game window, once your pick some power-ups tank icon changes, if you will pick rocket, tank icon with change with rocket indicator on it, if you will pick gatling cannon your tank will have gatling cannon mounted on it, etc.
P.S. Pleas use power-ups in the game wisely, sure they have great firing power, but if you will use them incorrectly you will end up destroying you own tank. Make sure that you know what you are doing.