Although we don't know all of the details yet; Treyarch developers have talked about certain elements of Black Ops 2 Zombies in recent interviews. To quote Treyarch's Mark Lamia, it will be the "most ambitious zombies effort yet".
As you can see, the trailer doesn't really give us too much information. There seems to be a large focus on the bus, which could mean that the bus will be used by players to travel in between map points (or dare I say, in between maps). It also looks as if the human race has had to endure some sort of Zombie Apocalypse, as burnt-out cars litter the landscape and smoke fills the air. The teaser lets us know that a full zombies reveal for Black Ops 2 will be released on the 26th of September.
A few hours before Treyarch released the teaser trailer, they uploaded the screenshot below with the title "The Calm Has Ended". Looks a lot like some sort of apocalyptic wasteland.
The image above was released by the official Call of Duty website. As you can see, it's a woman with a shotgun thrown over her shoulder and a zombie head dangling from her hand. Nobody actually knows if she's going to be in the game or not. If she is; she will be in fact, the first playable female character in a COD game (Correction: As somebody else pointed out, she won't be the first playable character because Sarah Michelle Gellar featured in the previous game).
During the past several months, several Treyarch developers have candidly spoken about porting the zombies game mode over onto the main multiplayer engine. According to them, this will allow for much more possibilities, which include:
Come November, players will be treated to larger Zombie maps and worlds. Having the game mode play on the main multiplayer engine will allow them to achieve such a feat.
They will be doubling the amount of players that can play co-op together. In the previous game, you could only party it up with 3 other players. This time around, you'll be able to team up with 7 people.
The multiplayer engine also allows for twice as many zombies, which means that players will find it difficult not to be overrun by my hoards of flesh eating members of the undead. Personally, I can't wait to see the mayhem and carnage that more players and scores of the walking dead are going to cause.
On the 27th of August, the official COD Facebook page posted a picture of the Nuketown sign with blood all over it. This means that Nuketown will definitely be a zombies map. Notice the counter!