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.
Ohio. 1  The Big One (Bryan, OH) 2  Van Wert County Fairgrounds (Van Wert, OH) 3  Tracy's Country Flea Market (Van Wert, OH)  Niekamp Farm Market (Celina, 1  OH) Darke County Steam Threshers - York Woods (Greenville, 2  OH) Cross's Campground (Camden, 3  OH) Pleasant Treasures (Hamilton,  OH) Covered Bridge Antique Mall (Cincinnati, OH)