Cup Qualifying Explained

In a perfect world the fastest 43 cars that show up for NASCAR qualifying on any given week would start the race. However in order to reward the teams that show up week in and week out NASCAR has guaranteed starting spots and provisionals available.

The Guaranteed Starters

Starting with the 2005 NASCAR NEXTEL Cup season NASCAR guaranteed the top 35 teams in car owner points a spot in the starting lineup. This is to encourage teams to run all the races instead of just a few and reward the teams that do show up every week.

This means that teams safely in the top 35 are only qualifying to determine where they will be on the grid and are in no danger of going home without racing. This sometimes allows these teams to spend their practice time working on race setups. This can pay off on race day, but could cause them to start deeper in the field.

Provisionals

After the top 35 in owner points take their guaranteed spot the seven fastest non-guaranteed drivers get into the field based on their qualifying time.
The seven drivers that get in on time will start wherever their times normally would put them in the order. So if a non-guaranteed driver runs quickest in qualifying he still gets to start on the pole ahead of the guaranteed, but slower, drivers.
This leaves one final spot which is known as "The Champions Provisional." This final 43rd starting position is reserved for any former NASCAR Champion that didn't qualify for the race any other way (by points or on time.)

If there is no driver eligible for the Champions Provisional then that spot goes to the eighth fastest driver that is not guaranteed a starting spot based on points.
Some Exceptions To The Rules

The most obvious exception to all of this is the Daytona 500. The Daytona 500 follows its own qualifying process that is unlike any other race on the NASCAR schedule.

Another exception has to do with the all-important car owner points. Through the first five races of the year NASCAR uses the car owner points from the previous season. Beginning with the sixth race of the year NASCAR switches to the current season's car owner points to determine the guaranteed starters.

And finally, what does NASCAR do when it rains or snows or for any other reason qualifying is cancelled? In that case NASCAR lines up the top 42 drivers by car owner points. Then the Champions provisional is still available to a former Champion not in the top 42. If there isn't an unqualified past Champion then the next driver in points gets the last starting spot.