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.
City of Wilmington Department of Finance. 800 North French Street. Wilmington, Delaware 19801. The City of Wilmington City of Wilmington - Tax Assessor & Finance Officer Office is located at 800 North French Street, at the Louis L. Redding City/County Building building in Wilmington, Delaware. City of Wilmington is a locality in New Castle County, Delaware. Unlike many other areas where property taxes are collected on a county level, City of Wilmington has its own tax assessor's office. City of Wilmington