big daddies are nothing when you have a grenade launcher!
There are these people called Splicers that attack you with hooks or pipes or anything they can get their dirty hands on, and they are fairly ugly creatures. Then there are the Big Daddies, dressed in bulky scubadiving gear. A little girl accompanies the Big Daddies most of the time, and she carries something called ADAM, which makes it possible for you to buy more spells. The spells are powered by another thing called EVE, and it can be found around the levels. You can buy weapon upgrades as well as more ammo in vending machines with the gold found on your victims.
When you kill the Big Daddies, you have a choice of harvesting the little girls for 160 ADAM, or rescuing them from the evil of the city and receiving 80 ADAM. Your choice defines the rest of the game, so choose wisely. I would highly recommend this game to anybody who likes FPSs.