The Great Plains consist of the extensive grasslands that spread across the heartland of North America.  They extend from southern Canada to central Texas, and from the eastern slope of the Rocky Mountains east to the Mississippi River Valley. 
The pre-contact population of the Plains was probably rather small, but when horses became available after about 1700 (Spanish), groups on the Plains expanded in population while groups from other regions moved onto the Plains.
The western portion of the Plains is a vast area of grassland.  This is due to the rain shadow effect of the Rocky Mountains – relatively little rain falls on the Plains, making it not suitable for trees.