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.
Cuyahoga County (about 24 miles in Cleveland, OH) Other public-use airports nearest to North Royalton: Columbia (about 15 miles in Columbia Station, OH) Medina Municipal (about 15 miles in Medina, OH) Schmeltzer (about 15 miles in Akron, OH) Amtrak Station near North Royalton: 13 miles: Cleveland (200 Cleveland Memorial Shoreway)