The ATP molecule basically functions as the power source for the cell. Sometimes it is compared to a battery, which is a pretty fair analogy, but it is more mobile and flexible in its functions than a battery is.
The acronym ATP stands for adenosine triphosphate, which is just a description of the molecule's chemical components; an adenine attached to a ribose sugar, and three phosohate groups. What's important here are the phosphates; these are groups of phosphorus and oxygen which have the ability to store large amounts of chemical energy in their bonds, and release that energy under controlled conditions. The more phosphates, the more energy that can be stored, which is why triphosphate is the energy carrier rather than diphosphate (ADP), its metabolic precursor and product.
One if the curious things about ATP and related molecules is their chemical similarity to DNA. It is not clear if they are actually related in evolutionary terms, and if so, which one preceded the other.
ATP is constantly being generated and broken down inside the body, and it appears in miscellaneous roles in various biological systems, but is most prominent within the metabolic system.
ATP, known scientifically as Adenosine triphosphate, is basically energy within cells. ATP stores and transports energy in the cells, usually in the mitochondria. Energy is released by hydrolysis (carbohydrates being broken down into sugar molecules), which eventually results in forming ADP (adenosine diphosphate) that absorbs the energy and recharges the phosphate group and ATP.