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.
Will County, IL. In 1997 Wilmington is 159 years old. Originally called Winchester, shortly after its founding, the name was changed to Wilmington when it was learned there was another Winchester in Illinois. Wilmington is often called the Island City. There is a large island, in the Kankakee River, within city limits.