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.
Lisbon, Ohio. Lisbon is a village in Center Township, Columbiana County, Ohio, United States. The population was 2,821 at the 2010 census. It is the county seat of Columbiana County. Lisbon is located in the Salem, OH Micropolitan Statistical Area, as well as the southern regions of the greater Mahoning Valley.