From my frontal lobe, Wikipedia:
Under the separation of powers created by the United States Constitution, the United States Congress has the sole power of the purse and responsibility for appropriating government funds. Like other bills, appropriations must be passed by both the House of Representatives and the Senate. Upon passage of a final version by both houses, they go to the President of the United States. If the President signs the bills, they become law. If instead the President vetoes them, they go back to Congress, where the veto can (in rare instances) be overridden by a two-thirds vote of both houses.