There's a blog post from Fred Wilson that I've found to be most insightful:

http://www.avc.com/a_vc/2010/08/what-a-ceo-does.html

The main point is this:

"A CEO does only three things. Sets the overall vision and strategy of the company and communicates it to all stakeholders. Recruits, hires, and retains the very best talent for the company. Makes sure there is always enough cash in the bank."