A storm is coming. In this storm: people, and also demons and elves and aliens and a tank. They're all having a big fight over who is the best assortment of five Blizzard characters in a semi-symmetrical battle with a broad selection of special conditions. In this, as in pretty much all lane-pushing games, the premise of Heroes of the Storm makes basically no sense.
No matter, because it'll be out soon—meaning you can be too busy playing it to worry about why any of it is happening. Blizzard has announced that Heroes of the Storm will go into open beta on May 20, before its full launch on June 2.
Heroes of the Storm pulls characters for Warcraft, Starcraft and Diablocraft. Thirty Heroes are planned for the games launch, with seven battlegrounds to fight over. For more details, you can watch Tyler's interview with Dustin Browder, or watch Tom Marks explain the five things you need to know (about Heroes of the Storm; not just in general).