A phosphoester bond is a bond between the phosphorous atom of a phosphate group and an oxygen atom. Phosphoester bonds should not be confused with phosphodiester bonds, which consist of two phosphoester bonds between a phosphate group and two oxygen atoms.
Phosphoester bonds are very important in biochemistry as they are essential for the phosphodiester linkages that form the sugar phosphate backbone of two nucleic acids: DNA and RNA. Phosphoester bonds are strong covalent bonds, so the nucleic acid backbone does not break easily. However, there are enzymes that are called phosphodiesterases. These enzymes break the phosphodiester linkages in nucleic acids.