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.
Seville, Ohio. Homefacts City Report. Seville is located in Medina County, OH. The population is 2,095, making Seville the 6th largest city in Medina County. There are 2 public schools in Seville with an average Homefacts rating of A-. The total crime rate for Seville is low, and there are 15 registered sex offenders residing in the city.