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. The City operates as a statutory (non-charter) city subject to the general statutes of the State, which are applicable in all municipalities in the State. In addition, the City may exercise all powers of local self-government under Article XVIII, Section 3, of the State Constitution.