Adenosine triphosphate (ATP), the energy currency or coin of the cell, transfers energy from chemical bonds to endergonic (energy absorbing) reactions within the cell. Structurally, ATP consists of the adenine nucleotide (ribose sugar, adenine base, and phosphate group, PO4-2) plus two other phosphate groups.

A 2-D stick view of the structure of ATP. 

A cartoon and space-filling view of ATP. Image from Purves et al., Life: The Science of Biology, 4th Edition, 

Energy is stored in the covalent bonds between phosphates, with the greatest amount of energy (approximately 7 kcal/mole) in the bond between the second and third phosphate groups. This covalent bond is known as a pyrophosphate bond.