Couplings are used to connect two joints of tubing or casing pipe together. Couplings are short length of heavy wall pipe with the same threads on each end.
Pup joints are used to adjust the height of full length tubing or casing strings. They are also used to adjust the depth of downhole tools.
Tubing is pipe used for the transportation of crude oil and natural gas from an oil or gas layer to the surface affter drilling is complete. Casing is a large-diameter pipe that serves as the structural retainer for the walls of oil and gas wells, or wellbore.