ATP is a molecule belonging to the family of nucleotide triphosphates. It contains a purine base called adenosine (hence the A), a five carbon sugar called ribose and three phosphate groups (hence the TP). The five carbon sugar can also be deoxyribose when ATP is used as a building block for DNA.
One of the main functions of ATP is to be a source/storage of energy that is used to aid in many cellular functions. The bonds among the phosphate groups are considered to be "high energy bonds" and their hydrolysis (or breakdown) releases energy. The breakdown of the first phosphate results in the formation of ADP (adenosine diphosphate) + phosphate. Further breakdown of the two remaining phosphates results in the formation of adenosine monophosphate (AMP) + phosphate and then adenosine + phosphate. Most cellular reactions that require energy do so by utilizing ATP as a coenzyme and the energy needed is obtained from the hydrolysis of ATP to ADP + phosphate.
ATP can also be used by a family of enzymes collectively called kinases to add phosphate groups to other molecules. This is also done by the hydrolysis of ATP to ADP + phosphate but the phosphate, instead of being liberated, is added to another molecule.