O=C-OH | R (where R is a hydrogen or organic group)
or any molecule containing such a functional group.
As the name implies, they are acids and react with bases to form carboxylate salts, in which the hydrogen of the -OH group is replaced with a metal ion. Thus, acetic acid (vinegar) reacts with sodium bicarbonate (baking soda) to form sodium acetate, carbon dioxide, and water:
Some carboxylic acids include
HCOOH formic acid (found in insect stings, formic refers to ants)
CH3COOH acetic acid (found in vinegar)
CH3CH2COOH propanoic acid
C6H5COOH benzoic acid (sodium benzoate, the sodium salt of benzoic acid is used as a food preservative)