A submarine is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term most commonly refers to a large, crewed, autonomous vessel. They may be operated by nation's navy/coastguard, or used for civilian purposes. Examples include nuclear-powered submarines ballistic missile submarines, and guided missile submarines. They are occassionally referred to as "boats" by crew members.
The Ohio-class submarine is a class of nuclear-powered, ballistic missile submarines utilized by the United States Navy, of which 18 of both the ballistic and guidance missile varieties were in usage. They were developed in 1976, and utilized 14 Trident II missiles, of which the total amount of each submarine contained 50% of the thermonuclear arsenal for the United States.
The Permit-class submarine was a class of nuclear-powered fast attack submarines utilized by the United States Navy during the 1960s and 1970s until replaced by the Sturgeon-class and Los Angeles-class, although they weren't fully retired until 1994. They were conceived from a study in 1956 by Admiral Arleigh Burke, the Chief Naval Officer at the time.