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, Ohio is located about 45 miles away from Cincinnati and about 50 to 60 miles away from Columbus. The mayor of Wilmington is David Raizk. Wilmington is prodominately Republican. It is a farming community and right now is suffering from DHL moving.