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.
Sandusky is a city in Northwest Ohio and county seat of Erie County, on Ohio's north coast along Lake Erie between Cleveland and Toledo. It is perhaps best known for Cedar Point Amusement Park, home to some of the world's tallest and fastest roller coasters. It is one of Ohio's prettiest downtowns and is very clean.