Demon's World (aka "Horror Story" and "Hora Story") is a simple platform game where you scroll horizontally thru hordes of evil incarnations. (There are actually two different versions, see note at bottom**)
You are always sporting a weapon to fire and you can change which weapon it is with a floating power-up that comes by often. The power-ups usually starts out as a constantly changing letter - and you have to try to time it to grab the letter (and thus the power-up) you want. Sometimes, after a while, the letter stops changing and simply becomes a static letter for you to grab that particular weapon (if you want to). Some of the weapons are: missiles, arrows, lasers, wide angle, etc. The wide angle is the only one that can shoot anywhere but straight horizontal. It is therefore highly recommended as many of the enemies float/fly while you run along the ground.
Of course, the other thing you can do is jump (this is a platform game after all). And more importantly, you can double jump. There are many bonuses and power-ups and enemies and places to go that you can only get to by managing a successful and well timed double jump (simply push the jump button again at the zenith of your first jump).
There also is a functional third button which is essentially just a repeating fire button.
The game has some other interesting additions. In the first level, there are balloons you can jump and grab to help you float down slower - and thus blast more airborne uglies. There are also many crevices, clouds, platforms, evil-doers, structures and the like that you have to jump on, over and around and blast away in order to live.
The other type of power-up (besides the changing weapon) - is the simple "P". Get three of them and you get a shield. Your outfit changes color and you can take one hit without dying.
The graphics are nothing special, but the theme is fun. You get to battle every monster you've ever heard of: Frankenstein, Dragons, Demons, Hockey-Masked slashers, vampires, etc. The game can be very challenging, so be glad you can continue ad infinitum.
**There are two versions of this game. One as I have just described and another Japanese version with demons and monsters of Japanese folklore and a completely different level design. After playing both, I like the Japanese themed game better. A pic from it is below...