Low density polyethylene tubing is the type used for transporting water and other food products. This type of tubing is resistant to cracking and is flexible. It is also holds up in different temperature extremes.
High Density polyethylene tubing is the type used in laboratories for scientific experiments and testing. It is also used for food handling. It's not as flexible as the low density tubing but does withstand corrosive chemicals better.
Polyethylene is a polymer made of hydrogen and oxygen. A polymer is a combination of many molecules strung together. The contents of the polymer determine its properties.