http://en.wikipedia.org/wiki/United_States_Naval_Special_War...

"Marcinko was the first commanding officer of this new unit, which was first called MOB 6 (Mobility 6) and Sixth Platoon. Eventually the unit was dubbed SEAL Team Six. At the time there were only two SEAL teams. Marcinko named the unit SEAL Team Six in order to confuse Soviet intelligence as to the number of actual SEAL teams in existence."

And what makes them so special:

http://en.wikipedia.org/wiki/United_States_Naval_Special_War...

"Candidates are put through a variety of advanced training courses led by civilian or military instructors. These can include free-climbing, advanced unarmed combat techniques, defensive and offensive driving, advanced diving, and Survival, Evasion, Resistance and Escape (SERE) training. All candidates must perform at the top level during selection, and the unit instructors evaluate the candidate during the training process"

In in short - they're the best of the best. Also, they haven't been defunct since 1987. They were renamed the United States Naval Special Warfare Development Group. The Army equivalent is Delta Force.