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 ranks in the upper quartile for Population Density when compared to the other cities, towns and Census Designated Places (CDPs) in North Carolina. See peer rankings below. The primary coordinate point for Wilmington is located at latitude 34.2092 and longitude -77.8858 in New Hanover County.