According to the Perfect World Entertainment site, Battle of the Immortals will be a "2.5D MMORPG" that will feature things such as 5 different playable classes; PvP including Territory War; PvE content with both an auto-quest-navigation system and high-end raids; pets; mounted combat; and more. The story is said to cover everything from Norse mythology to the Qin Empire, which should be interesting to see.
If your curiosity is piqued, why not pop on over to the PWI site and check out the story & concept art. Alternately, you could just skip to the meatier features or gear screenshot post if story-lines aren't your thing. They also hint that they'll be posting about the beta for Battle of the Immortals soon, which we'll be keeping an eye out for.