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.
Columbiana County is a county located in the U.S. state of Ohio. As of the 2010 census, the population was 107,841. The county seat is Lisbon. The county name is derived from the explorer of the Americas, Christopher Columbus. Columbiana County comprises the Salem, Ohio Micropolitan Statistical Area, which is also included in the YoungstownâWarren, OH-PA Combined Statistical Area.