Clear the island of Bombolia by blasting all the creatures to kingdom come! Skillfully pilot your balloon with the Arrow Keys (or AWSD keys) and press the Space Bar to drop your bombs, then wait for them to explode. Aim and timing is crucial and as you progress the enemies get tougher, faster and smarter - luckily you can upgrade your arsenal with different bomb types and powerups. Make sure to keep an eye on the radar telling you where enemies are too... and whatever you do, don't run out of bombs or time! Should you want to pause for whatever reason, then push either "P" or "Escape".
Here's some lovely screenshots for you: