ATP is adenosine triphosphate, the *only* cellular energy source for your body (a molecule). ATP is the most significant product of aerobic (with oxygen) cellular respiration.
The basic equation:
Carbohydrate (glucose) + Oxygen ===(goes to, through detailed metabolic processes)==> Carbon Dioxide + Water + ATP. ATP is *THE* form of energy your body uses in its processes.
You can generate ATP from lipid peroxidation as well, notably.