The Midwest is of great debate. You can break it up into two halves: the Great Plains and the Great Lakes/Rust Belt. West of the Mississippi is flat with agriculturally-based economies, east is more varied terrain with industrial economies in the cities.
Colorado and Wyoming are considered Mountain West, with Idaho, Utah, Montana, and eastern Washington State.
This is from Joel Garreau's The Nine Nations of North America. I'd rename and tweak a few things, but it's an interesting and more accurate take on regional breakdowns.