Polymer means "many parts". A polymer is a substance composed of complex molecules. Theses molecules are composed of long chains of simpler units called monomers. Almost all the things we call "plastics" are polymers, and it is the longchain molecule that gives them their useful characteristics. Stated very simply, polymerizing means the chemical linking of monomers (simple compounds) to form the wide variety of polymers (with longchain molecules) which are the basis of the modern plastics industry.