Wikipedia's write up on the purpose and honor of Arlington:
Arlington National Cemetery, in Arlington, Virginia is a military cemetery in the United States, established during the American Civil War on the grounds of Arlington House, formerly the estate of the family of Robert E. Lee's wife Mary Anna (Custis) Lee, a descendant of Martha Washington. The cemetery is situated directly across the Potomac River from Washington, D.C. and near The Pentagon. It is served by the Arlington Cemetery station on the Blue Line of the Washington Metro system.
More than 300,000 people are buried in an area of 624 acres (2.53 km2). Veterans and military casualties from every one of the nation's wars are interred in the cemetery, from the American Civil War through the military actions in Afghanistan and Iraq. Pre-Civil War dead were reinterred after 1900.
Arlington National Cemetery and United States Soldiers' and Airmen's Home National Cemetery are administered by the Department of the Army. The other National Cemeteries are administered by the Department of Veterans Affairs or by the National Park Service. Arlington House (Custis-Lee Mansion) and its grounds are administered by the National Park Service as a memorial to Lee.