Plastic is the common name given to the products of a particular type of chemical reaction, the polymerisation process. In this reaction, a huge number of small molecules are linked together into incomprehensibly long chains, which curl about and cross-link to one another to form a usually flexible material with a high resistance to tearing. Different starting materials, additives and reaction conditions result in a wide range of products with different physical properties.
Plastics can be broadly split into two groups, those that consist of long strands (thermo softening) and those that also contain short cross-links (thermosetting). Thermo softening plastic will deform when heated and can be remoulded into new shapes. Thermosetting plastics are much stronger, but once they have been formed into a shape, they will hold that shape indefinitely, and if heated they will merely burn.
The great advantages of plastics are that they're relatively cheap to produce1, they're easily moulded, they're light, and because of their inert nature and smooth surface, they're safe for use in contact with food, for they neither rot nor create house bacteria.