Polymers are long chain molecules that occur naturally in living things and can also be made by chemical processes in industry. Plastics are polymers, so polymers can be extremely useful.
Some polymers are made via condensation polymerisation.
In condensation polymerisation, a small molecule is formed as a by-product each time a bond is formed between two monomers. This small molecule is often water.
An example of a condensation polymer is nylon.