SSK - Hunter-Killer Submarine

SSK was the United States Navy hull classification symbol for a diesel-electric submarine specialized for anti-submarine duties. SS indicated that the vessel was a submarine, and the K suffix that it was a hunter-killer. The United States Navy does not currently operate any submarines of this type, and so the designation is inactive.

The start of the Cold War in the mid-1940s and the threat of Soviet submarines in the Atlantic led several Western navies to build or adapt submarines to specialize in hunting other submarines. The changes included streamlining to make them quieter and improved acoustic sensors. This type of vessel was given the classification SSK in United States service. The changes were eventually incorporated into all submarines, allowing the SSK role to be subsumed into the regular attack submarine role and the classification became obsolete.

http://en.wikipedia.org/wiki/SSK_(hull_classification_symbol)