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.
Tuscarawas County (TUS-kÉ-RAH-wÉss) is a county located in the eastern part of the U.S. state of Ohio. As of the 2010 census, the population was 92,582. Its county seat is New Philadelphia. Its name is a Delaware Indian word variously translated as old town or open mouth.