Blizzard's Warcraft setting used to be pretty straight high fantasy - elves, orcs, dwarves, various flavors of monsters. but after a couple of decades of strategy games and MMO action, the World has developed its own flavor. Much like Terry Pratchett's Discworld, Azeroth is sort of fluid: its fantasy tropes can be teased and prodded into more or less any genre of storytelling, so long as you don't object to seeing the occasional zombie or minotaur in your high-stakes heist plot.
So it is with Mean Streets of Gadgetzan, which brings humorous elements of noir gangster and hard-boiled detective genres into the Hearthstone multiplayer card game. The titular Gadgetzan is a fantasy city where crime is rampant and gold is king. Sure, it's a little jarring to see orcs in 1920s-style flapper dresses and Murlocs shooting at the local constabulary from a Neolithic sidecar motorcycle. But that's par for the course in WoW and Hearthstone these days, and fans wouldn't have it any other way.
The new expansion adds over 130 new cards to collect, many of which can be wielded by any of the game's deck-building heroes. There are also new tri-class cards, which can be used by three different hero classes according to their "crime family" alignment. (You can see the complete collection here.) Given how the game's competitive nature can be altered by a single card or creative combinations of new strategies, they're sure to shake up multiplayer battles in new ways. There are also new decorative cardbacks and game boards. Gadgetzan packs can be purchased in the usual manner, via in-game gold that can be topped up with real money.
(Here's a tip: if you download Hearthstone from the Amazon Appstore, you can use Amazon Coins to pay for new decks, which will carry over to your Hearthstone account on any Android, iOS, or PC hardware. If you've been slowly collecting Amazon's branded currency through those generous promotions and don't know what to spend it on, this might be a good way to get rid of it.)