During the American Civil War, the Union was a name used to refer to the federal government of the United States, which was supported by the 20 free states and five border slave states. It was opposed by 11 southern slave states that had declared a secession to join together to form the Confederacy. The Union has often been referred to as “the North”, both then and now.
The Union Army was the land force that fought for the Union during the American Civil War. It was also known as the Federal Army, the U.S. Army, the Northern Army and the National Army. It consisted of the small United States Army (the regular army), augmented by massive numbers of units supplied by the Northern states, composed of volunteers as well as conscripts. The Union Army fought and eventually defeated the smaller Confederate States Army during the war which lasted from 1861 to 1865. About 360,000 died from all causes; some 280,000 were wounded.
Learn more in the WikiPedia Union Army