Ok, I'm am getting so confused on these fittings. Flared fittings, O ring fittings, AN fittings, pipe fittings.

K, so I know what FPT and MPT is. So, what does NPT stand for?? And, what size NPT would fit a 3/4" FPT??? What the hell size is a #6 compared to a #8 AN fitting?? Someone enlighten me please!!!

Dean