PEX is cross-linked polyethylene and it is not ordinary polyethylene pipe.
PE stands for polyethylene and X for cross-linking.
The material's chemical abbreviation is PE-X.
There is PEX specifically made for potable (drinking) water.
There is PEX made for under floor piping and there is PEX specifically made for radiation type heating.

Cross-linked Polyethylene, often referred to as PEX, is a thermoelastic material made by cross-linking polyethylene.

Its strength at temperatures ranging from below freezing up to almost boiling makes PEX an ideal piping material for in-house hot and cold water installations, under floor heating and de-icing applications.

PEX pipes can be manufactured in three different ways, depending how the cross-linking of the polymer chains is being made.

PEX was developed in the 1950's. It has been used for pipes in Europe since the early 1970's, and has been gaining popularity rapidly over the past decades.
In 1960s PEX tubing has been in use in many European countries for plumbing, radiant heating and snow melt applications since that time. PEX was introduced in the United States in the 1980s, and has seen significant growth in market demand and production.
PEX’s flexibility and strength at temperatures ranging from below freezing up to 200 degrees Fahrenheit makes it an ideal piping material for hot and cold water plumbing systems, hydronic radiant heating systems, snow melting applications, and, even ice rinks and refrigeration warehouses.
The flexibility of PEX allows it to be supplied in coils meaning installations under the slab can be made with a single, continuous length without the need for fittings under the slab. PEX is not affected by concrete, or chemicals in concrete (it is commonly encased in concrete for radiant floor heating). PEX, however, must be sleeved when penetrating a concrete slab

Although the high temperature resistance of PEX makes it particularly suitable for hot and cold interior plumbing applications, it also makes an excellent underground water service piping. It can be installed using the same fittings recommended for copper tube sized SDR-9 polyethylene tubing.