Heroes of the Storm is Blizzards take on the MOBA (multiplayer online battle arena). You get to use the heroes and villains from the Blizzard games like Warcraft, World of Warcraft, and StarCraft. The game officially launches Tuesday 6/2/2015. Players who join the first week get some special rewards.
According to the official website
Soon the Nexus will be full of heroes battling it out for glory and to help celebrate, we’re sharing free XP! Use the XP to unlock your favorite Master Skin, or secure level 30 to play Hero League. Players who log in to the Americas region between the hours of 12:01 AM PDT on June 2nd and 11:59 PM PDT on June 9th will receive a 25% XP boost.