Stanford attempts to assign housing in as fair a manner as possible, because some residences are much better than others.  To accommodate various students' needs, there are three rounds of assignments that take place.  The earlier rounds tend to give you better housing, since the last two rounds are basically a way of filling in vacancies.  In other words, you'll just be filling up whatever is available.

Round 1: The Draw
This is what most people think of when they think "housing assignments".  This is the lottery based system that tries to assign housing to students in as fair a manner as possible.  If you want housing next year, you should enter this round.  If you do not meet the deadline to enter this round but still want housing, you absolutely need to enter the waiting list.  The exception is for those of you participating in preassignments.

Round 2: Waiting Lists
If you meet any of the following conditions, you will be participating in the waiting list:

• Entered the draw, but did not get assigned to housing because your number was bad
• If you're coming back from abroad and need housing
• If you requested reassignment because you did not like the housing you were assigned by the draw.

If you do not enter the waiting list by the waiting list deadline, you may not be guaranteed housing for the next year.  If you entered the draw and were unassigned, you will automatically be placed in the waiting list, so you do not need to worry about the deadlines.

Round 3: Walk-ins
If you are still unassigned after the waiting list round, then you need to participate in the walk-in round.  This takes place once the quarter starts.  Most of the time the housing office will find a vacancy for you to live in.  Until then, you might have to stay with one of your friends, or potentially one of the graduate or community residences with vacancies.