Karl Dönitz used the term Rudeltaktik to describe his strategy of submarine warfare—Rudeltaktik translates best as "tactics" of a "pack" of animals and has become known in English as "wolfpack" (Wolfsrudel), an accurate metaphoric, but not a literal translation.
U-boat movements were controlled by the Befehlshaber der Unterseeboote (BdU; English translation: "Commander of Submarines") much more closely than American submarines, which were given tremendous independence once on patrol. Accordingly, U-boats usually patrolled separately, often strung out in co-ordinated lines across likely convoy routes (usually merchants and small vulnerable destroyers), only being ordered to congregate after one located a convoy and alerted the BdU, so a Rudel consisted of as many U-boats as could reach the scene of the attack. With the exception of the orders given by the BdU, U-Boat commanders could attack as they saw fit. Often the U-Boat commanders were given a probable number of U-Boats that would show up, and then when they were in contact with the convoy, make call signs to see how many had arrived. If their number were sufficiently high compared to the expected threat of the escorts, they would attack.
Convoy escorts and anti-submarine aircraft, November 1941
Although the wolfpacks proved a serious threat to Allied shipping, the Allies developed countermeasures to turn the U-boat organization against itself. Most notably was the fact that wolfpacks required extensive radio communication to coordinate the attacks. This left the U-boats vulnerable to a device called the High Frequency Direction Finder (HF/DF or "Huff-Duff"), which allowed Allied naval forces to determine the location of the enemy boats transmitting and attack them. Also, effective air cover, both long-range planes with radar, and escort carriers and blimps, allowed U-boats to be spotted as they shadowed a convoy (waiting for the cover of night to attack).