The functional group of a carboxylic acid is called a carboxyl group, which consists of a carbonyl group (C=O) and a hydroxyl group (−OH) attached to the same carbon atom. The general structure is R-COOH, where R represents a variable organic group.
The functional group of an ester is called an ester group. It consists of a carbonyl group (C=O) and an alkoxyl group (−OR) attached to the same carbon atom. The general structure is R'-COOR, where R' represents an organic group different from R.
In summary, the carboxylic acid functional group is the carboxyl group (-COOH), while the ester functional group is the ester group (-COOR).