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.
detailed map of Wilmington and near places. Welcome to the Wilmington google satellite map! This place is situated in New Hanover County, North Carolina, United States, its geographical coordinates are 34Â° 13' 32 North, 77Â° 56' 42. West and its original name (with diacritics) is Wilmington.