As of this writing, Bioshock is THE best PC game I have ever played. Actually, it's tied with another game that I love that also came out in 2007 although it's really unfair to compare the two since Bioshock is indeed a strange, strange game. This visually spectacular first person shooter (FPS) with survival horror overtones and elements of an RPG provides you with such a mesmerizing and engrossing environment that you are often tempted to simply hang out in the game and just play around with stuff instead of dogmatically proceeding through completing all the game's objectives.
But once you step out of the bathysphere and begin picking your way through the bowels of the city, you discover the truth: Rapture is in shambles. It is in fact nothing but a leaky, broken-down dystopia, ransacked, raped and rife with a screwball citizenry.
By means of a radio you find in the bathysphere, you are immediately contacted by a man named Atlas who pledges to keep you alive and begins guiding you through what Rapture has become and coaching you on how to survive in it. It appears that the citizens of Rapture have been undergoing rampant genetic modifications as a result of recent scientific discoveries. Initially meant to better mankind, these genetic splices have had some egregious side effects on the once good people of Rapture. The citizens —now known as 'Splicers'— have become a band of xenophobic, wrench/gun/bomb-toting lunatics wandering aimlessly about babbling away to themselves with their tempers all on a hair trigger. And into this powder keg you proceed...
You are tasked with completing goals which are fed to you one-by-one, typically by Atlas. Completing each goal allows you to progress deeper and deeper into the game. You find keys, you blow open passageways, you fetch special items and the like. You always have access to a map of the current level you're on which normally shows the location of your current goal by displaying a star. There is also an in-game quest arrow (thankfully optional) that constantly points you in the direction you are supposed to proceed.
The key element of gameplay in Rapture that resembles that of an RPG is acquiring and using 'plasmids' which are genetic modifications you can make to your character to give you access to special superpowers. These plasmids allow you to do things like shoot electricity or fire out of your hand, or throw objects using telekinesis or control the minds of your opponents (and even turn them against one another).
Many plasmids are simply found and you equip them as you see fit as you go along. But for the most part, plasmids are acquired at special plasmid vending machines known as "Gatherers Gardens". Gatherers Gardens take a serum known as Adam as currency and the Adam is obtained from strange little girls wandering about Rapture known as Little Sisters. However each Little Sister is always protected by a fierce monstrosity dressed in a diving suit known as a Big Daddy (featured on game box cover).
Two or three Little Sister/Big Daddy pairs appear on each of the game's levels. Defeating the Big Daddies in order to get Adam from the Little Sisters and then using that to purchase and equip more powerful plasmids is integral to advancing easily through the game.