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, North Carolina. Wilmington is a port city and the county seat of New Hanover County in coastal southeastern North Carolina, United States. The population is 112,067; according to the 2010 Census it is the eighth most populous city in the state.