The three parts of adenosine triphosphate, or ATP, are the adenosine group, a ribose group similar to RNA and the string of three phosphate groups.
The adenosine group is composed of two rings of bonded carbon, nitrogen and hydrogen. The ribose is a sugar, the same sugar which is the basis for RNA, composed of oxygen and hydrogen bonded to a ring of carbon.
The critical part of ATP are the phosphate groups in its tail, especially the third. When ATP loses its third phosphate group to another organic molecule, the energy released powers a cellular mechanism. Indeed, this process is the major way every organism on Earth uses energy.
In cellular respiration, the most common way life forms obtain energy from food, cells use the highly energetic reaction between oxygen and glucose to gain energy. This energy is then used to attach a phosphate group to adenosine diphosphate, or ADP. Adenosine diphosphate is the molecule left over after ATP releases its stored energy.
Not all organisms use oxygen to create ATP all the time, and some never do. The two alternatives are fermentation and anaerobic respiration. In fermentation, the first step in processing glucose, glycolosis, is extended to generate as much energy as possible. In anaerobic respiration, cells use compounds other than oxygen to perform the same role.