A bill becomes a law when signed by the president. If the president vetoes a bill, Congress can override the veto by a two-thirds vote of both houses.
There are many reasons for a president to reject legislation. For example, although the president may be supportive of a bill's main purpose, he may decide that it contains unacceptable riders. If the president does not sign or veto a bill within ten days, the bill becomes law. On the other hand, the bill is dead if Congress adjourns within this ten-day period. This is known as a pocket veto.
In 1996, Congress gave the president line-item veto power, which meant he could reject specific spending items within a larger bill. The U.S. Supreme Court struck down this attempt to increase presidential discretion two years later, however, in a 6-3 opinion written by Justice John Paul Stevens.