Polyester is a category of polymers which contain the ester functional group in their main chain.
However! the term "polyester" as a specific material most commonly refers to polyethylene terephthalate (PET), which can be made in two ways, and both are done in chambers that feature strong vacuums and high temperatures. # The first requires dimethyl terephthalate and ethylene glycol. You make it this way, and the process will liberate methanol--which you have to capture, store and figure out something to do with. (In most cases, the "something to do with it" is to mix it with terephthalic acid and feed it back into the process.) # The other is to use terephthalic acid and ethylene glycol. This requires more heat, but it produces no methanol--a very important thing when you consider how dangerous methanol is. No one wants it around if they're not using it.
Some polyesters also include naturally-occurring chemicals, such as in the cutin of plant cuticles, as well as synthetics such as polycarbonate and polybutyrate.
It's a type of polymer that can be made from both natural and man-made sources. It's typically used to make a fabric that is used in clothing, home items, belts, yarn, rope, bottles, tarps, canoes, finishing for wood, and many other uses.