It's a puzzle/platformer sort of thing in the vein of the original Prince Of Persia set in this science fiction/fantasy world. The main character isn't a buff, tuff or mean killing machine, he's actually a delicate, fragile little green alien who will die instantly at the hand of any sort of harm. By the end of the game, you'll have died many, many times. You'll be shot, crushed, blown up, cut apart, ripped apart, mauled; eaten, gassed… the list goes on. So the developer thought it would be fair to give you infinite lives. It fits the game well. It's still completely difficult, but now you'll be able to take risks here and there and know you don't have much to lose.
Instead of fighting all the weird and brutal monsters you encounter, the game is designed for you to use your brains to work around them. Everything you encounter in the game is better equipped, faster, stronger and outright nastier. But they all have heads full of rocks. The really clever thing about the game is the ability to talk to other characters using gamespeak - you can greet (Hello) and bark orders (Follow Me, Work, Wait, Stop That!) - a lot of the game's puzzles center around guiding friends through traps and puzzles and using teamwork to open up a new path.
At points you'll have to sneak around, surveying the environments. In others you'll have to run from enemies. In others you'll have to jump over bombs and dodge falling rocks. There are moments where you have to think fast and make quick decisions. There are moments where you have to take in everything you know before your next move. The balance of scenarios and the impeccable pacing are definitely something to behold. You're never caught off guard by the random appearance of an enemy. It's very, very clever design.
It's excessively hilarious and the design, art direction and production values are top-notch. It's so engaging and addictive to play.
It's one of the games I grew up on and actually recieved critical awards over Final Fantasy VII in the year it was released. The second one is even better.
http://www.gamefaqs.com/console/psx/review/198224.html - some reviews of the first game.
Both games for $5 US all up is a bargain.