What is the Stanley Parable? Well that depends on which Stanley Parable you're asking about.

The Stanley Parable (classic)

The first Stanley Parable is a Half-Life 2 mod released in 2011 which eschews the typical run and gun action in favor of story telling. You play as the titular Stanley, an office worker who spends his days watching a screen for instructions on what button to press. Stanley's job is mind numbingly boring but he loves it all the same. Until one day, the instructions stop coming. The game begins with Stanley going to investigate. Stanley is accompanied by the Narrator, a disembodied voice who explains things and tells you what to do whenever you have a choice. Of course you would never deliberately derail his carefully crafted plot, right?

Yeah, never.

The fun in the Stanley Parable comes not from the environment which is little more than rooms and hallways cobbled together from existing Half-Life 2 assets nor from the game play which is best described as a dull walking simulator. No, the fun of the Stanley Parable is listening to the Narrator (voiced by Kevan Brighting) react to your choices. His posh British accent combined with his passive aggressive attitude toward player agency makes the Stanley Parable really memorable by emphasizing one of the parts of gaming that is generally hidden: linearity.

There are almost no alternate paths, no choices, no places where you can strike out and alter the outcome of the story in either of the Half-Life games. These days many games tend to take the opposite approach, dropping players in an open world filled with opportunities. The Stanley Parable takes neither option but rather gives the players a series of explicit choices whether to follow the narration with reward of either seeing where the plot goes or listening to the Narrator whine, threaten, or beg Stanley in to cooperating. This whole exercise is made especially ironic by every path being merely another branch on the dialog tree.

The Stanley Parable HD Remix

While the original Stanley Parable is certainly a wonderful curiosity it's short enough to explore every ending in one sitting and the reused models from its parent game gives the entire game a cobble together appearance. To remedy both of these problems the humble mod was raised to the status of a full game when it was publish by Galactic Cafe Studios in 2013. The HD Remix consists of entirely new environments; offices that look like offices, probably a dozen maps, a secret disco, the works. On top of the graphical improvements it also features more than twice as many endings, some of which are extremely difficult to reach.

Unlike the original, HD Remix is a commercial release and costs fifteen bucks at the time of writing. In my opinion this is a bit high but Steam sales being what they are you can probably get it for less.

The original Stanley Parable can be found for free here though you'll need Half Life 2 to play it. The HD Remix, its Demo, and trailer can be found here.