The United States has three kinds of nuclear missiles. One batch, called ICBMs (Intercontinental Ballistic Missiles) are big missiles based on silos in the United States that fly from the United States to targets anywhere in the world. Some are deployed from aircraft. Some, called SLBMs (Submarine Launched Ballistic Missiles), are deployed on Ohio Class ballistic missile submarines.
The trouble is that the U.S. has more nuclear weapons than it needs. The solution has been to convert four nuclear missile submarines (starting with the first in the class, the Ohio) to SSGN status (conventional guided missile submarines). The Ohio has just been converted to carry 154 Tomahawk land attack cruise missiles (the same kind of missiles carried on U.S. cruisers (up to 127 missiles total, usually in a mix of types) and destroyers (up to 96 missiles total, usually in a mix of types)), as well as 66 Navy SEALs (i.e. commandos).
The idea behind having a submarine, rather than a surface warship, carry these missiles, of course, is that the submarine can get closer to the target, while being less of a target itself, leaving less time for a response to a launched missile. Being nuclear powered, it is also less dependent on other ships for logistics support.