Okay, so by a certain completely objective quality measure, Super Crate Box is the worst game ever. Let's look past that for the time being.
Here's the gameplay: this is a single-screen platformer. A variety of enemies fall from the sky and wander downward. If they reach the bottom of the screen, they reappear at the top in a powered-up state. Touch an enemy, and it's game over; there are no extra lives. This means that games tend to last on the order of seconds, not hours.
On the screen somewhere is a crate. Your goal is to pick it up. When you do, you get a random weapon and discard your old one, and a new crate appears.
You can shoot at enemies to kill them, and you often do just to keep the number of threats manageable, but your final score is simply the number of crates you collected. Thus, even if you get a really nice weapon, it is not a matter of "how do I use this weapon to kill the most baddies" but rather "how do I use this weapon to clear a path to the next crate... okay, now how do I use this NEW weapon to get me to the NEXT crate..."
All in all, a fun retro romp that's worthy of getting some play during your next coffee break as an alternative to Minesweeper. After all, there's something to be said for any game where a double-digit score feels like an accomplishment.