The basis of any of the plastic molecules are polymers. A polymer is a substance that consists of a large molecules (macromolecules), with weight from a few thousand to several million. The molecules of the polymers consist of a large number of identical parts connected to each other. These units are called monomers. Natural polymers is cellulose (the main component of plant cell walls and starch: they consist of long chains of glucose molecules connected to each other. The polymer is natural rubber.
Plastic is a man - made polymers. Called them so because it is plastic materials when heated or under pressure they are melted and can be formed, and after cooling or hardening retain their shape as plasticine. The structural units (monomers) plastics are low molecular weight (small) molecules, which emit from oil, coal or natural gas. So, of ethylene gas by a chemical reaction of polymerization (i.e., to create long chains of molecules of ethylene) get well known plastic polyethylene used to make the packages (the prefix "poly-", which is present in the names of polymers, means in Greek "a lot"):
Pellets of polyethylene
To make plastics of various properties they add dyes, antioxidants, fillers and other additives.
Prikumskaya plastics plant (LLC "Stavrolen"), in Budennovsk