Pipe thread types 

There are two types of pipe threads: taper pipe threads and parallel pipe threads.

division	External thread	Internal thread	combination
For tight coupling taper pipe threads Example: Prevent water and gas from leaking	R(PT) Taper	Rc(PT) Taper
		Rp(PS) parallel
Mechanical coupling parallel pipe thread Examples: Protection tubes for wiring cables, etc.	G(PF) parallel	G(PF) parallel

The symbols in parentheses in the above table are the symbols used in the Former JIS

Taper pipe threads

Tapered pipe threads are used for water pipes, gas pipes, and other areas where leakage of liquids or gases through gaps in the threads would be a problem.
Its application is called tight bonding.
A taper is a shape that gradually becomes thinner or thicker.
I think it is easy to understand if you imagine the red cone used at construction sites.

Tapered thread combinations for pipes

External threads are one type of tapered, but there are two types of internal threads: tapered and parallel.
Whether you attach the R tapered external thread to the Rc tapered thread or the Rp parallel internal thread,
It is getting thicker and thicker, so it will definitely stop somewhere.
 

Parallel pipe threads

On the other hand, if dense resistance is not required, such as a protective tube for a wiring cable, parallel pipe threads are used.
Its application is called mechanical coupling.
 

Combination of parallel pipe threads

Both the external and internal threads are parallel. The G of the parallel thread for the pipe is also the external thread and the internal thread, and the thread symbol is G.
 

Basically, it is based on English, but only R is said to be derived from German for some reason.
It is said that it comes from the word "Rogebinde", which means pipe screw in German.

(ANSI) ASME B1.20.1

(ANSI) ASME B1.20.3

(DIN) DIN10226-1,-2