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.
Montgomery County, Ohio. Montgomery County is a county located in the U.S. state of Ohio. As of the 2010 census, the population was 535,153, making it the fifth-most populous county in Ohio. The county seat is Dayton. The county was named in honor of Richard Montgomery, an American Revolutionary War general killed in 1775 while attempting to capture Quebec City, Canada.