From the National Park Service site:
The Civil War Soldiers and Sailors System is a computerized database containing very basic facts about servicemen who served on both sides during the Civil War. The initial focus of the CWSS is the Names Index Project, a project to enter names and other basic information from 6.3 million soldier records in the National Archives. The facts about the soldiers were entered from records that are indexed to many millions of other documents about Union and Confederate Civil War soldiers maintained by the National Archives and Records Administration.
Other information includes: histories of regiments in both the Union and Confederate Armies, links to descriptions of 384 significant battles of the war, and other historical information. Additional information about soldiers, sailors, regiments, and battles, as well as prisoner-of-war records and cemetery records, will be added over time.
The Civil War Soldiers and Sailors System (CWSS) is a cooperative effort by the National Park Service (NPS) and several other public and private partners, to computerize information about the Civil War. The goal of the CWSS is to increase the American people's understanding of this decisive era in American history by making information about it widely accessible. The CWSS will enable the public to make a personal link between themselves and history.
Also available on Ancestry.com as "U.S. Civil War Soldiers, 1861-1865".