Dishonored is a quiet epic of strategy and assassination.
You play Corvo, the bodyguard of the Empress of Dunwall - a faded alternate history of criminals and disease. While Dunwall is cut off by the outside world in an attempt to contain its plague, the Empress is assassinated, Corvo framed and his mistress's daughter, Emily, taken. The night before he's to be executed Corvo is led to escape and from there he/you must use his/your skills in the art of murder to get Emily back and restore the rightful royal lineage.
It's tempting to say 'Dishonored is a stealth game', but that doesn't really do it justice. You can play it as a stealth game, but it's open enough that it really does encourage you to play the way you want to play. The environments are pretty substantial, for starters. It's not quite an open world, but the chunks of Dunwall you navigate are substantial enough that you rarely feel funneled in a particular direction.
So, faced with a given mission - assassinating a noble you're assured is a bad bloke, for example - you'll have to decide whether you want to find a long way round to their location, see if you can make it across the rooftops, look for a low route (possibly through tunnels) or go straight for them. And, if you go straight for them, will you try to stick to cover and remain unseen, stealthily kill everyone in your way or (and you'll need to be tough to pull this off) go straight down the middle, fighting everyone as you go?
The choices you make in acquiring supernatural powers (provided by the mysterious, corporeal Outsider) have a genuine impact on how you're able to tackle things, too. Powers such as Dark Vision and Blink work better for quieter, evasive play styles where Bend Time (bullet time, really) or Windblast are more offensive skills.
Pushing your decision making even further is the Chaos system. Where other games have offered you 'good' or 'evil' in-game choices, the Chaos system doesn't place value judgements on your actions. Rather, killing enemies willy-nilly produces more corpses, more rats and more disease, potentially leading to riots and affecting how NPCs interact with you. Bethesda says that your Chaos rating also affects the scenes you encounter and factors such as the number of enemies in the game. You really do have to watch your step in Dishonored, because there's every chance a bad decision will come back to haunt you...
With this Definitive Edition not only do you get the core game, you also get all of its expansions - Dunwall City Trials, The Knife of Dunwall, The Brigmore Witches and Void Walker's Arsenal.