When a bill is passed by both houses it is sent to the President to be signed. The President may sign it, veto it, or take no action.
When a President comments on and refuses to sign a bill it is known as a veto. A vetoed bill may return to Congress for reconsideration, where Congress may override it by a 2/3 vote in both houses.
If the President does not act within 10 days the bill automatically becomes law. However, if Congress adjourns during those 10 days the bill is vetoed in a "pocket veto."
Public Papers of the President of the United States