How a Bill becomes a Law

In the United States, we have two chambers of Congress called the House of Representatives and the Senate. An idea to make a law can start in either chamber (with one exception: any bill that authorizes spending has to originate in the House. There are ways to bypass that, but let's not complicate things). Here is the process by which a bill has to go through:

1. Introduced in Congress - Self Explanatory. Usually a bill will be brought up and it will be voted to..
2. Sent to a Committee - There are several committees in both chambers that consist of people that are most knowledgeable about a topic. They will debate and make amendments (if necessary) to the bill and vote for it to return to the floor of the chamber it was introduced in.
3. Voted on by the Chamber it was Introduced in - This is a full vote by either all 435 Representatives or 100 Senators. If passed, the bill will move to..
4. Introduced in Other Chamber - the other chamber that did not vote on it. The process of a vote or sent to committee occurs again. Alterations may be made by the other chamber to the bill before passage. If the bill is altered, it goes to a...
5. Conference Committee - Members of the House/Senate meet to make changes that will ensure the bill passes both the House and the Senate in the exact same text. Once that happens, the bill is sent to the President.
6. Presidential Decision - If the President likes the bill, it will be signed into law (or slight alterations can be made via Executive Signatures). If s/he doesn't like the bill, s/he can veto it, sending it back to Congress. It takes a 2/3rds majority vote in both chambers to override this veto. S/He has ten days (as authorized by the Constitiution) to make a decision. If s/he does nothing, then the bill automatically becomes law.

There are technicalities like a "pocket veto" (if Congress is not in session, the President can let the bill wait 10 days and it'll become vetoed without a chance to over-ride), but this is the general concept of how a bill becomes a law in the United States.