If you click on this link and scroll to the bottom you will see all the cars and tracks that come included with your initial iRacing Membership (18 cars, 25 tracks): http://www.iracing.com/membership/
We provide all of the cars and tracks needed to run official Rookie series. Any additional tracks/cars will need to be purchased. We do this so you can pick and choose which cars and tracks to get, and don't end up paying for things you may not want or use. You only need to purchase them one time. You can test any car on any track as long as you own it. You can participate in a hosted race on any car and any track as long as you own it also.
Below is a current list of all base content:
Cars:
Cadillac CTS-V Racecar
Dirt Street Stock
Dirt UMP Modified
Formula Vee
Global Mazda MX-5 Cup
Kia Optima
Legends Ford '34 Coupe & Rookie
Lucas Oil Off Road Pro 2 Lite
Pontiac Solstice & Rookie
Radical SR8
SCCA Spec Racer Ford
Street Stock
Toyota GR86
VW Beetle
VW Beetle Lite
VW Jetta TDI Cup
[Legacy] Dallara DW12
[Legacy] NASCAR Truck Chevrolet Silverado - 2008