An ester is a chemical compound that is formed by reacting an alcohol with a carboxylic acid. In its simplest form, it consists of a hydroxyl group (-OH) from the alcohol and a carbonyl group (C=O) from the carboxylic acid, connected through an ester linkage or R-COOR', where R and R' represent alkyl groups. Esters are commonly found in nature and have various applications in industries such as fragrances, plastics, and pharmaceuticals.