Collectively, Polyethylene is the most common plastics in the world, but it's classified into three types: High-Density, Low-Density and Linear Low-Density.

All the plastics are polymer but all the polymers are not plastic which means plastic is a sub-division of polymer. Polymers are uniform molecules formed by small monomers and plastics are long-chain molecules formed by large monomers. Polymers can be either natural or synthetic but plastics are synthetic materials.

Polymers are derived from petroleum, and their low cost has its roots in the abundance of the feedstock, in the ingenuity of the chemical engineers who devised the processes of manufacture, and in the economies of scale that have come with increased usage.

Polyethylene, polystyrene, and acrylic are examples of plastics formed by addition polymerization. These polymers are often thermoplastic in nature: they can be heated and made soft and then hardened when cooled. They are easily processed, reprocessed, or recycled.