"A bill or joint resolution that has been vetoed by the President can become law if two-thirds of the Members voting in the House and the Senate each agree to pass it over the President's objection. The chambers act sequentially on vetoed measures: The House acts first on House-originated measures (H.R. and H.J. Res. [House Joint Resolution]), and the Senate acts first on Senate-originated measures (S. [Senate] and S.J. Res. [Senate Joint Resolution]). If the first-acting chamber fails to override the veto, the other chamber cannot consider it. The House typically considers the question of overriding a presidential veto under the hour rule, with time customarily controlled and allocated by the chair and ranking Member of the committee with jurisdiction over the bill. The Senate usually considers the question of overriding a veto under the terms of a unanimous consent agreement."
CRS Report for Congress, RS22654
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html