The ester group is characterized by being an internal functional group, that is, it does not appear in the ends of the molecules. It presents two oxygen atoms bound to the same carbon atom, one through a double bound and the other one through a single one (R1-COO-R2). It is a neutral polar group, that is, it does not possess acid or basic properties. It can establish hydrogen bounds, acting as a H acceptor.
It is formed by the reaction of a carboxyl and a hydroxyl functional groups. Putting it in a simple way, it is the result of the substitution of the hydrogen atom of a carboxyl group by an organic portion.
It can be identified by infrared sepctroscopy (it presents a band between 1730-1750 cm-1).