Prairies are natural communities dominated by grasses on mineral soil.
Low shrubs may dominate locally; trees may be present, but less than ten percent of the area has a tree canopy *.   Six prairie subclasses are recognized in Illinois:

   Prairie (tallgrass prairie on silt-loam soils)
   Sand Prairie
   Gravel Prairie
   Dolomite Prairie
   Hill Prairie (including loess and glacial drift hill prairie)
   Shrub Prairie

Illinois Prairie Types & some local examples

prairie gravel dolomite
dry dry prairie dry gravel prairie dry dolomite prairie
moist dry mesic prairie dry mesic gravel prairie
Santa Fe Prairie
dry mesic dolomite prairie

wet mesic prairie
Wolf Road Prairie
mesic gravel prairie
Santa Fe Prairie
mesic dolomite prairie
Lockport Prairie
wetter wet-mesic not recognized in Illinois wet-mesic dolomite prairie
wettest wet prairie not recognized in Illinois wet dolomite prairie
Romeoville Prairie

* - White and Madany 1981, quoted from the Lower Des Plaines River Area Assessment, Vol. 3 - Living Resources, published by the Illinois Dept. of Natural Resources' Critical Trends Assessment Program