The Art Director sets the standard that everyone on the project is working towards, and works closely with the Art Manager for the project they happen to be working on, as well as the recruiting team to build the best art team they can. Their main focus is to manage the team as a whole, making sure different branches of the project are working along the same lines as each other.
Art Directors are responsible for all of the artist within a project and work directly under the company and are often in close contact with the client for whichever project the team is working on.
In some ways I would say it's one of the most creative roles you could have, as you have to solidify the visions from all the different sections within the project into something tangible.
Obviously to become an Art Director you would need to have good organisation and management skills as well as an open and creative mind. Communication skills would be invaluable in this role, and although this is speculation on my part, I would say that an Art Director would have to be able to keep a cool head in any situation - it is a very powerful title to have and without a cool head a whole project could collapse around one arguement.
As for "How is art direction in Games similar to or different from Film, for example?" I will come back to this question.