An executive order is a written directive of the President of the United States of America. It looks something like this:

Executive Order: (topic)

By the authority vested in me as President by the Constitution and the laws of the United States of America, including (list a lot of pertinent laws here), I hereby (do something).

(details of the order)

(more details)

(name of the president),
THE WHITE HOUSE,
(date).

George W. Bush's executive orders are released by the press secretary and put online at http://www.whitehouse.gov/news/orders/. Eventually, they are printed in the Federal Register.

Generally speaking, executive orders are only effective at telling executive agencies, such as the Department of Defense or the Central Intelligence Agency, what to do. The President couldn't pass an executive order to disband Congress or overturn a Supreme Court ruling, for instance.

Governors also issue executive orders for state governments to follow.