Short answers, pulled from the story.
The Army of the Potomac was formally created on the 26th of July, 1861, when Major General George B. McClellan merged the Departments of Northeastern Virginia, Washington, Pennsylvania, and the Shenandoah under a single command. It was disbanded on the 28th of June, 1865.
Six generals commanded the Army of the Potomac: Irvin McDowell, George B. McClellan, Ambrose Burnside, Joseph Hooker, and George G. Meade, with Meade holding command the longest from the 28th of June, 1863, through the 28th of June, 1865.
The Union force that preceded the Army of the Potomac, then called the Army of Northeastern Virginia under Brigadier General Irvin McDowell, fought and lost the First Battle of Bull Run. The defeat prompted the reorganization that produced the formal Army of the Potomac under McClellan.
At Gettysburg on the 1st of July, 1863, the Army of the Potomac defended Cemetery Hill against Lee's Army of Northern Virginia. The I, II, and III Corps suffered catastrophic losses; one corps commander, Reynolds, was killed, Sickles lost a leg, and Hancock was badly wounded.
Soldiers in the Army of the Potomac held sharply divided views on whether the war's purpose was to preserve the Union or to end slavery. Officers loyal to General McClellan and Copperhead Democratic politics clashed with others who supported emancipation, and some officers who voiced dissent faced court-martial.
The Society of the Army of the Potomac was a veterans association founded in 1869 by men who had served in the army. It held annual reunions for more than fifty years, with its final gathering taking place in 1927.