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.
If trying to research your Clinton County, Ohio, roots from afar, here are some pointers. The library has Wilmington, Ohio, newspapers on microfilm; however, contents are not indexed. Staff can do a limited search of the microfilm if you provide a name and specific date. The library does not have copies of Clinton County records.