Consider the example of polyethylene terephthalate (PET or "polyester"). The monomers which could be used to create this polymer are ethylene glycol and terephthalic acid:
HO-CH-CH-OH
and
HOOC-CH-COOH
In the polymer, there are two structural units, which are
-O-CH-CH-O-
and
-CO-CH-CO-
The repeat unit is
-CH-CH-O-CO-CH-CO-O-