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.
Hamilton, OH. Sponsored Topics. Hamilton is a city in Butler County, southwestern Ohio, United States. The population was 62,447 at the 2010 census. It is the county seat of Butler County. The city is part of the Cincinnati metropolitan area. The city's mayor is Patrick Moeller.