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.
This article is about the city. For the unincorporated community in Champaign County, see Middletown, Champaign County, Ohio. For the community in Crawford County, see Middletown, Crawford County, Ohio. Middletown is a city located in Butler and Warren counties in the southwestern part of the U.S. state of Ohio. Formerly in Lemon, Turtlecreek, and Franklin townships, Middletown was incorporated by the Ohio General Assembly on February 11, 1833, and became a city in 1886. The population of Middletown as of the 2010 census was 48,694.