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 is the principal city of the Wilmington Metropolitan Statistical Area, a metropolitan area that includes New Hanover and Pender counties in southeastern North Carolina, which has a population of 263,429 as of the 2012 Census Estimate. Wilmington was settled by European Americans along the Cape Fear River.