Normally (during non-Cheney era's) this is the second most important position in the White House. The White House Chief of Staff is responsible for organizing the president's day and determining who gets to meet him. He is the president's "gatekeeper."
Obama has selected Rahm Emanuel serve as his chief of staff, who as of this moment has yet to accept it. He has a reputation as being very aggressive and opinionated. Qualities which which should serve him well in setting the president's agenda, assuming he does not feel constrained in the role.
Here is a profile from Rolling Stone, and one from the Telegraph.
