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. 1  is a neighborhood in the city of Los Angeles in the Harbor region of Los Angeles County. 2  The neighboring communities are Carson, Harbor City, Long Beach, San Pedro and West Carson. 3  Wilmington is one of the 272 neighborhoods in Mapping L.A., The Timesâ resource for crime, neighborhoods, demographics and schools.