Black Nipples & Fittings are commonly used for natural gas applications in

Copper Drainage, or DWV (Drain-Waste-Vent) fittings are commonly used in residential

Copper Pressure fittings are typical in residential and commercial water supply

PEX fitting system using Reinforcing rings and cold expansion fittings.

Short description of a Pex Crimp fitting here is where the information goes blah

Here's some info on Pex Cold Expansion fittings.  This is the category description.