Bobby M's website (
http://www.brewhardware.com/fittings) has a pretty straight forward description of what each letter means.
My advice - take the time and draw out ALL your hoses, pumps, kettles, and anything that needs to be connected together. Then write 2 letters next to each connection point. M or F for the pipe thread, and M or F for the camlock. So if you needed a MALE (M) NPT and MALE (M) Cam Lock, you'd order an "F" type camlock. If you wanted it to be Female (F) NPT and M Cam Lock, you'd order the A.
What I did to make things slightly easier - all my hoses are Female camlocks. Some are Type B and the hose slips over the threads (though could go with Bobby's "Big C"). Some are Type D with a street elbow (Bobby's BL kit)... but all the hoses have the female type camlock. That way I knew all the fittings on the pumps or on the plate chiller or on the kettles would be male type cam lock, and then it was only a matter of figuring out if I needed Male NPT or Female NPT.
Of course, I had to order about 2 or 3 times and got a few extra pieces, but that's because I'm a "Do as I say, not as I do" type person...
The only down side of camlocks - they get a little hot. The advantage of putting the female camlock on the hose - you can disconnect it using the pull ring and hold the hose by that - the rings tend to stay cooler.
-Kevin