For instance if I want to know what plane N5FNAA is and I know that the plane is a Boeing 757, go to the 757 page at:
and then if you have a "find" button on your keyboard, look for the middle thee numbers (between the "n" and the "aa" ex: find "5FN" and it should scroll right down to the aircraft. N5FNAA is N174AA.
Based on the tail #'s that have been converted so far in this thread, I'm assuming if the number after the "N" is a "5" then it's a 757, if the number is a "3" it's a 737 and if it's a "4" it's a MD-80 or variant.
Hope that helps a little.