Short answers, pulled from the story.
The Army of the Shenandoah was a Confederate field army created to defend the Shenandoah Valley of Virginia from Union attack during the early months of the Civil War. It was built from militia and volunteer companies assembled at Harpers Ferry and eventually comprised four brigades of infantry and the 1st Virginia Cavalry under J. E. B. Stuart.
General Joseph E. Johnston commanded the Army of the Shenandoah. He replaced Thomas J. Jackson as commander at Harpers Ferry on the 15th of May 1861. Johnston led the army through its transfer to Manassas Junction and held overall command at the First Battle of Bull Run because he outranked General P. G. T. Beauregard.
Johnston's brigades marched to the Piedmont station and boarded trains on the Manassas Gap Railroad, arriving at Manassas Junction one brigade at a time because only one train was available. The artillery and cavalry marched overland. The final units arrived on the afternoon of the 21st of July, the day of the battle.
After the First Battle of Bull Run, the Army of the Shenandoah was merged with Beauregard's Army of the Potomac into the Department of Northern Virginia. The combined force took the name "Army of the Potomac," and Johnston retained command with Beauregard serving as second in command until his transfer to the western theater.
Jackson received the nickname "Stonewall" while defending Henry House Hill at the First Battle of Bull Run on the 21st of July 1861. His brigade held its position as fighting raged across the hill, and the name "Stonewall" subsequently became attached to his brigade as well.
Patterson commanded Union forces from the Department of Pennsylvania tasked with keeping Johnston's army occupied in the Shenandoah Valley. When Patterson called off his advance after Stuart's cavalry screened Johnston's position, Johnston was free to transfer his entire army to reinforce Beauregard at Manassas Junction. Johnston's troops arrived in time to help decide the battle on the 21st of July 1861.