Greeley, Colorado is the town where Kenny, Kevin and Karen McCormick, (and later, Eric Cartman) are sent to live with foster parents, the Weatherheads, in "The Poor Kid". It seems to be a relatively rural town, with many pastures and a large number of cattle. Their baseball field is seen in a much earlier episode, "The Losing Edge".
It is described as pretty much the "exact opposite of Hawaii", much to Cartman's disappointment. A lot of the kids in town wear torn or dirty clothes and have dirty faces.