First, a bill must pass both houses of Congress by a majority vote. After
it has passed out of Congress, it is sent along to the President. If the
President signs the bill, it becomes law.
The President might not sign the bill, however. If he specifically rejects
the bill, called a veto, the bill returns to Congress. There it is voted on
again, and if both houses of Congress pass the bill again, but this time by a
two-thirds majority, then the bill becomes law without the President's
signature. This is called "overriding a veto," and is difficult to do because
of the two-thirds majority requirement.
Alternately, the President can sit on the bill, taking no action on it at
all. If the President takes no action at all, and ten days passes (not
including Sundays), the bill becomes law without the President's signature.
However, if the Congress has adjourned before the ten days passes and without a
Presidential signature, the bill fails. This is known as a pocket veto.