Plastic is a commercial name for a group of materials that while being processed, can be pushed or formed into almost any desired shape and then retain that shape. Plastics can be cast, molded, or pressed into an unlimited variety of shapes. They are one of the most used materials on a volume basis in industrial and commercial life. Plastics are on par with metals, wood, and ceramics and are essential to the needs of virtually the entire spectrum of business. Plastics, properly applied, will perform functions at a cost that other materials cannot match.

Most plastics can be classified as either thermoplastic or thermosetting materials. Thermoplastic materials can be formed into desired shapes under heat and pressure and become solids on cooling. If they are subjected to the same conditions of heat and pressure, they can be reprocessed into new shapes. Thermosetting materials are like concrete, once processed and shaped, they cannot be reshaped. Today, the vast majority of plastics are thermoplastics.

Plastics are made up of polymers. Polymeric materials are characterized by long chains of repeated molecule units known as "mers". These long chains intertwine to form the bulk of the plastic. The ways in which the chains intertwine determine the plastic's macroscopic properties. Typically, the polymer chain orientations are random and give the plastic an amorphous structure. Amorphous plastics have good impact strength and toughness. Examples include acrylonitrile-butadiene-styrene (ABS), styrene-acrylonitrile copolymer (SAN), polyvinyl chloride (PVC), polycarbonate (PC), and polystyrene (PS).

If instead the polymer chains take an orderly, densely packed arrangement, the plastic is said to be crystalline. Crystalline plastics share many properties with crystals, and typically will have lower elongation and flexibility than amorphous plastics, and better chemical resistance. Examples of crystalline plastics include acetal, polyamide (PA; nylon), polyethylene (PE), polypropylene (PP), polyester (PET, PBT), and polyphenylene sulfide (PPS). Advances in chemistry have made the distinction between crystalline and amorphous less clear, since some materials like nylon are formulated both as a crystalline material and as an amorphous material.