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.
Delaware County is a county in the U.S. state of Ohio. As of the 2010 census, the population was 174,214. Its county seat is Delaware. The county was formed in 1808 from Franklin County, Ohio. Both the county and its seat are named after the Delaware Indian tribe. Delaware County is included in the Columbus, OH Metropolitan Statistical Area. U. S. President Rutherford B. Hayes was born and grew up in Delaware County.