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.
In June of 2010, the Clinton County, Ohio Port Authority took ownership of the Wilmington Air Park. It now operates and manages over 1,900 acres surrounding the two-runway, fully functional Part 139 airport. Nearly 3 million square feet of office, warehouse and hangar space are included in the airport property.