Pipe Fittings:
Fitting manufacturers, in conjunction with the Society of Automotive Engineers (SAE) have established an industry standard for improvement in the operation of pipe threads.  This improvement resulted in the development of a new style of pipe fitting know as the Dryseal Pipe Thread.  All FittingConnection pipe threads are Dryseal American Standard Tapered Pipe Thread (NPTF). The metal-to-metal seal is formed by contact of the thread crest and root.

It is also important to note that nominal pipe sizes do not indicate either the I.D. or the O.D. or thread size of a pipe fitting. To accurately determine the correct pipe fitting size, (up to 1-1/4”) it is necessary to measure the diameter of the thread and subtract ¼”. For example, if you were to measure the O.D. of a ¾” male nominal pipe fitting, you would discover it to be 1”. Then you would subtract ¼” from the O.D., thus coming up with a ¾” male pipe fitting.

For more information on Pipe Connections see related links:
Pipe Connections

Tube Fittings:
There are three basic types of tube fittings: Flare, Flareless and Straight Thread O-ring. Tube fittings are designed to seal in one of two possible ways.   Straight thread O-ring fittings are designed to seal using a rubber O-ring between the mating components.  When extreme vibration is present, use Flareless or Straight Thread O-ring Fittings. 

For more informaton on Tube Connections see related links:
Tube Connections