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.
Lucas County is a county equivalent area found in Ohio, USA. The county government of Lucas is found in the county seat of Toledo. With a total 1,543.31 sq. km of land and water area, Lucas County, Ohio is the 1756th largest county equivalent area in the United States. Home to 435,286 people, Lucas County has a total 201580 households earning 41556 on average per year.