The U.S. Census Bureau's American Community Survey (ACS) provides annual demographic data including educational attainment, foreign born population, language spoken at home, household income, and housing characteristics.
When using the American Community Survey's data, it is best to use percentages rather than report actual numbers because the ACS is a survey of a small subset of the county's population.
Loudoun County Analysis
The U.S. Census Bureau's Decennial Census is conducted every ten years to count 100% of the county's population. Census 2010 data can be found at the Census Bureau's American FactFinder. Simply type in “Loudoun County” in the search box.