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.
The county seat for New Castle County is located in the Louis L. Redding City/County Building in Wilmington, which is where the City of Wilmington Assessors' Office is also located. You can direct your general inquiries to New Castle County with the contact info listed below.