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.
â1420 S. Wilmington St. Raleigh. In 2001, the County opened a newly renovated emergency and transitional shelter for homeless men that changed the way we offer services to the homeless population. The South Wilmington Street Center provides a tier of services both on- and off-site that are designed to lead men to self-sufficiency. The shelter provides some short-term emergency dorms with overnight shelter only, while other dorms offer longer-term shelter to men who engage in services.