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.
Local info of Wilmington in Middlesex county. Wilmington has 22,907 residents in 2010-2014, ranked #62 in Massachusetts state cities. Also Wilmington income, school, races, crime, weather, environment and other info / rankings.