A -COOH group. One of the oxygens is double-bonded to the carbon atom, making it a carbonyl group, and the other oxygen is single bonded to the carbon on one side, and single bonded to the hydrogen on the other. The remaining bond on the carbon atom is attached to the rest of the molecule.
Organic molecules containing carboxyl groups are an important, major group of compounds studied in the field of organic chemistry.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.