There are four Race Tournaments in Grand Theft Auto: San Andreas. When you enter the map marker for the tournament, select a race from the menu, which provides info about the length of the track, best time and position (if you've attempted it), and a small map of the course. There is no limit to the number of times you can attempt a race.
If your vehicle is destroyed, or you destroy an opponent's vehicle, the race is failed. If you exit your vehicle, you will have 25 seconds to re-enter it, or the race will be failed.
Each race is a different course. You must pass through a succession of markers, and you must pass through a marker to activate the next marker. Each race carries a reward of $10,000. Finishing all races gives the player a fresh $1,000,000 and percentage for 100% completion.
To obtain any vehicle used in any race, set a marker to where you start the race, then when you start the race, drive/fly towards the marker. Park the vehicle nearby, get out and fail the race, when you exit of the race select screen, look for where you left the vehicle, and it should be there.