Right from the overview on the website:
Welcome to Yahoo! Sports Fantasy Auto Racing, where you run your own fantasy team of real professional drivers.
Each week, before qualifying begins, you choose an eight-driver team from among three groupings of eligible drivers. Then, before the race, you select four starting drivers for your team.
So, the challenge is not only to pick the best team race in and race out, but also to manage your team in a way that produces the highest possible fantasy score.
Each driver can receive a maximum of 90 fantasy points during each race. The top 45 finishers receive points, with the winning driver getting the maximum 90, and other drivers receiving 2 fewer points for every spot they finish behind the leader (e.g., the second-place driver gets 88 points, the third-place driver gets 86).
You receive the total points earned by all active drivers on your team provided that they haven't already been active in nine races (see the Maximum Starts section for more information).
Bonus points are awarded to each of the drivers with the 4 best starting positions after qualifying. The top qualifier receives 10 bonus points, the second receives 5, the third receives 3 and the fourth receives 1. Because qualifying for Daytona 500 is broken up over 2 days, these bonus points are not awarded for the Daytona 500.
An additional 10 bonus points are awarded to each driver who leads at least 1 lap during a race. The driver leading the most laps during a race also earns another 10 bonus points. To earn these bonus points for your fantasy team, a driver must be on your team and active during the race.
If a race is officially canceled, no fantasy team receives any points for that race. All other scoring rules stay the same. If qualifying or a race is delayed or postponed, no points are awarded until it's completed. If qualifying is canceled and the starting grid is determined by Sprint Cup points standings, no qualifying bonus points are awarded.
The link to read about it:
http://help.yahoo.com/l/us/yahoo/racing/rules/