The first trailer for Starcraft II: Legacy of the Void has been shown off at Blizzcon, and you can check it out below. The trailer focuses on the single-player story, so it's all very epic and filled with cutscenes. More interestingly, Blizzard has released a multiplayer update video for each race, showcasing the new units in Legacy of the Void and the changes that will affect the old ones.
See it all below.
Starcraft II: Legacy of the Void is the final part in the Starcraft II trilogy. You can now sign up for the beta here.