Who are the Horde and Alliance?
The Horde and Alliance are the two sides battling for control of the world. In years past, they fought brutal wars against each other, but today, they have settled into a tenuous stalemate. Occasional skirmishes still erupt between the two sides, but open warfare is no longer raging across the continents. Still, the Horde and Alliance are hostile to each other, and members of one faction are never welcome in the cities of the other.
The eight races you can play in the game are split equally between these two sides.

Alliance: Humans, Dwarves, Night Elves, Gnomes
Horde: Orcs, Tauren, Trolls, Undead.

Will players have the ability to change factions (for example: from Horde to Alliance)?
No.

Can the same account have both a Horde and Alliance character on the same Realm?
On normal servers this is possible, but on player-vs.-player servers (PvP) it is not.

Can players trade with members of the opposite faction?
No. In general, you cannot trade with players of the opposing faction. However, you can place items up for auction at the neutral auction houses at Everlook, Gadgetzan, and Booty Bay. Players from either faction can bid on these items, meaning you can buy from opposing players at this neutral auction house.

Will Horde and Alliance characters have the ability to gain faction points, thus allowing them to utilize each other's cities?
You cannot switch sides. Thus, you cannot make use of the opposing faction's cities.