In what way is the functional group of an ester different from that of a carboxylic acid?

The presence of OH
## – CO-OH## is carboxylic acid functional group.
## -COO-## is ester functional group.
The presence of Hydrogen in ##-OH## form makes the carboxylic acids behave like inorganic acids. Esters on the other hand are less reactive compounds than Carboxylic acids.
Carboxylic acids are acids like ##HCl, H_2SO_4## etc. They react with bases to form salt and they turn blue litmus paper to red and have other properties of acids. They are more weaker than inorganic acids. (i.e.)These acids are weak in terms of corrosive ability and this is due to the fact that they are compounds with covalent bond and dissociate with a reaction which is reversible in NTP.
##R-COOH rightleftharpoons R-COO^(- )+ H^+##
This means that the above reaction is subject to Lee-Chatlier Principle and Common ion effect. Hence the mixture of carboxylic acids and their salts with bases form a solution which is of constant pH. This solution is called buffer solution. (e.g) Acetic acid and Sodium acetate.
The esters on the other hand are products of carboxylic acids and alcohols. They are of general formula ##R – COO- R’##, They are neutral compounds. Animal fat and vegetable oils are esters of carboxylic acids and glycerol. Even butter is butyric acid ester. They hydrolyse to form the original acid and the alcohol components.
This reaction is the one used in Soap manufacturing. They hydrolyse the oils with Potassium hydroxide and form potassium salts of the carboxylic acids and the glycerol is separated and used for other purposes like manufacture of moisturising creams etc. The potassium salts are soaps which is then made into a solid masses. This reaction is called Saponification reaction.