Though it includes the town of Shawnee proper, Shawnee Mission was created by the United States postal service to designate a region containing several towns inside Johnson County, Kansas. Other towns inside the area include Mission Hills, Overland Park (but not all of it), and Lake Quivira.
Not surprisingly, the area is named for the Methodist mission set up first in nearby Wyandotte County, then moved to Johnson County in 1839. Today, the area is part of the Kansas City Metropolitan Area and contains more people total than Kansas's largest city, Wichita.
Today, this area has a half-urban, half-suburban feel with lots of people living there, but spread out over a wide area. This means that you can find just about anything you're looking for in town if you're willing to do just a little bit of searching. And, of course, the locals have that friendly Midwestern attitude - they're only too glad to help.