In iRacing, there are a various number of surfaces that any driver in the sim may deal with on any given race day. These surfaces contain the smooth asphalt surfaces, the loose dirt and gravel surfaces and more. Not all surfaces are the same and drivers that race on these surfaces will have to be best prepared in order to master these surfaces.
The most common type of surfaces in iRacing is the asphalt where grip is plentiful but not as loose as dirt, gravel, or grass. Another surface type that is present in the sim is concrete where its a faster surface type than asphalt but wears down a driver's tires faster than asphalt. Finally, the dirt surface on the dirt oval and the gravel surface on the dirt road contain fast, speedy action but are both quite loose and can be quite tricky to get used to at first.
Also, be mindful of the run-off area on road courses as those feature a different surface type called AstroTurf, otherwise known as artificial turf, which is made to look like natural grass and has also little to no grip just like regular grass. Also on the run-off areas of many road courses in the sim, you will also find gravel traps which are designed for driver safety and minimizes the severity of such an impact between the car, the driver, and the outside walls and tire barriers.
While those are the main surface types, another surface that you may encounter depending on the situation or misjudgment would be the grass, otherwise known as regular turf or natural turf. This surface is as loose as the dirt and gravel surfaces but contains little to no grip at all which brings it a high level of carefulness and extra caution which is a necessity in order to traverse this quite unforgiving surface.
Get acquainted with these surfaces from time to time to know their strengths and weaknesses so you can use them to any driving advantage.