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.
- Significance: The Bidwell and Gallagher Factory has been identified by the Ohio Historic Preservation Office as the oldest existing industrial/commercial building in Sandusky County. It is located adjacent to the Sandusky River and contributed to the growth of the river port of Lower Sandusky, later named Fremont. - Survey number: HAER OH-38 Medium