Prior to World War I, Germany was looking for an edge. They couldn't take on England's Grand Fleet in a straight fight – especially with a full naval blockade that was in place at the start of the war.
The submarine really made its mark on Sept. 22, 1914, when the U-9, an older U-boat, sank three British cruisers in about an hour in the North Sea.
The most common of the U-boats in German service was the UB III coastal submarine. According to U-Boat.net, that submarine had a range of over 9,000 miles on the surface, and a top speed of 13.6 knots. When submerged, it could go 55 miles and had a top speed of 8 knots. It had four torpedo tubes in the bow, and one in the stern, and carried ten torpedoes with a crew of 34 men.
German U-boats in Kiel. U-20, which sank the Lusitania, is second from the left in the front row. (Library of Congress photo)
U-Boat.net notes that Germany built 375 U-boats of all types during World War I. Of those 375, 202 were lost in action during World War I. The German U-boats were quite successful, though, hitting over 7,500 ships. That said, it is arguable that German submarines also hurt Germany in the war overall, as opinion in the United States turned against Germany after the sinking of the Lusitania, and Germany's use of unrestricted submarine warfare brought The U.S. into the war.
Ultimately the U-boats were neutralized by the convoy system starting in June, 1917. At the end of World War I, 172 U-boats — some of which were completed after the war — were surrendered to the Allies.
The video below from the History Channel discusses Germany's World War I U-boats, and how they changed the shape of naval warfare.