Bubble Bobble is a simple arcade game in which you blow bubbles at the bad guys, then pop the bubbles by running into them and causing the bad guys to turn into fruit or candy. The level is complete once all the enemies are put into bubbles and popped.
If you have another person around, playing with two players makes the game easier and more fun. You essentially have twice as many lives but more importantly two players playing cooperatively at the same time.
There are various power ups throughout the game with different effects, from giving you special firing power, to skipping levels and more. One type of power up is the letters. There are six letters that will appear in bubbles: E (x2), X, T, N, D. Once you collect at least one of each and spell EXTEND on the left, you get a bonus life, skip to the next level, and a few points related bonuses.
The trick to riding the bubbles is to press jump again as soon as you land on them. This will be very useful when the levels start getting harder.
Here is a very useful page on power ups and bonuses.
The controls are shown on the first screen that pops up when you run the game. From here you can press N to accept the default controls and start the game or Y to change the controls. Additionally, F1 is pause.
Important Technical Note:
There are two downloads. The first has only the one player mode, and the second has only the two player mode. In the second, to play with one player you can just wait for one to die, then not continue with that player. To revive a player, press fire. It will cost a credit.