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.
Wilmington (in Los Angeles County, CA) Populated Place Profile. Wilmington (GNIS FID: 1661697) is a populated place LOCATED WITHIN the City of Los Angeles. In turn, the City of Los Angeles is located within Los Angeles County. Wilmington is located at latitude 33.78 and longitude -118.263. The elevation of Wilmington is 23 feet above mean sea level.