Here is the plain English of #header .callout: Select all elements with the class name callout that are decendents of the element with an ID of header. Here is the plain English of #header.callout: Select the element which has an ID of header and also a class name of callout. Maybe this graphic will make that more clear:
Wilmington, Ohio. Wilmington is a city in and the county seat of Clinton County, Ohio, United States. The population was 12,520 at the 2010 census.
The jurisdiction of the Stark County Health District is that of the 17 Townships, 13 Villages, and the cities of Louisville, North Canton, and Canal Fulton. 3951 Convenience Circle, NW Canton Ohio, 44718.