Always verify requirements with your water supplier.
PVC fittings are gray-colored (except schedule-40 transitions which are white). Nylon fittings are a somewhat translucent white color and generally more durable.
PVC, Nylon, and brass are suitable for most potable, cold water supplies and pressures. PVC, Nylon, and stainless steel fittings are lead-free. Brass fittings are not lead-free unless so stated in the description for the specific item. Galvanized steel is also suitable, but will corrode in some environments. Stainless steel may be required where connecting to a municipal water supply.
Each insert end has "barbs" or ridges which are angled to allow the barbed end to insert into the tubing but resist retraction. They also provide a surface against which hose clamps can compress the tubing to form a tight seal. Hose clamps (usually one or two per insert) are used to secure the fitting inside the line or tubing.