In March 1918, the Germans delivered an attack against the British 5th Army called Operation Michael. The Germans realized that the tactics used by both sides on the Western Front were inadequate, and devised new tactics. They split their forces into two types of units, trench divisions for defense and Stosstruppen (storm troops) divisions for offense. These Stosstruppen were the best men culled from all divisions (leaving the rest for trench divisions) and extensively trained.
The tactics, which were refined into the Blitzkrieg of WW2, consisted of:
Aufmarsch - rapid movement to the enemy's front, generally by train and motor transport.
Gefeschsstreifen - concentration against a narrow sector of the enemy's front, then
Schwerpunkt - center of gravity for the attack, to be made with great force but with a short artillery bombardment (unlike the literally days of bombardment previously used).
Einbruch - penetration of the enemy front which, if successful, would be followed up by additional forces in order to achieve
Durchburch - breakthrough, permitting the troops access to the enemy's rear, where they could employ
Falchen und Lüketaktick - the tactics of space and gap, avoiding the enemy's reserves and strong points as much as possible, hitting them where they were least able to defend, in order to press on and secure control of enemy lines of communication, while other troops undertook the
Aufrollen - rolling up of the tattered ends of the pierced enemy front, mopping up strong points and widening the gap that additional forces could move up to support the advancing spearheads in attaining
Keil und Kessel - literally "wedge and pocket," the encirclement of the enemy.
Operation Michael succeeded until the Aufrollen stage, when the British 2nd Army to the north and the French 1st and 3rd Armies to the south were able to threaten the German flanks and Allied reserve troops deployed in a new front line. Army Group Crown Prince Rupprecht advanced as much as 30 miles beyond the original front line before it was obvious that the attack had failed.
For more information, I recommend:
J. H. Johnson, 1918: The Unexpected Victory. London: Cassell & Co., 1997.
John Keegan, The First World War. New York: Vintage Books, 2000.
Correlli Barnett, Swordbearers: Supreme Command in the First World War. New York: William Marrow & Co., 1964 (specifically the section on Ludendorff).