Ester

An ester is a molecule that includes a carbon atom, double-bonded to an oxygen atom, and single-bonded to another oxygen atom that is bonded to a carbon atom. In other words, a carbonyl with an ether group on the carbon atom. The group can be written as COOR. They can be made by reacting a carboxylic acid with an alcohol with some acid. 
Esters are a very important functional group. They can be found in many substances. They are very common in fats and oils. They sometimes have a very strong, nice smell. They can be used to make perfumes. Polyesters are found in plastics.
Esters can also do many reactions. It is very easy to change the ether part of the molecule with another similar one. It is also possible to reduce the ester to a ketone or an alcohol, which can do many different reactions. Nucleophilic addition is also a good reaction, though a bit more difficult than with ketones.
There are many ways to find out if a molecule has an ester group. Infrared spectroscopy gives very sharp signals, which are different from other carbonyls. Carbon NMR spectroscopy has similar characteristics.