ATP
Adenosine Triphosphate

Adenosine Triphosphate (ATP) is a chemical that all cells use to allow reactions to take place.  ATP is the "energy currency" of the cell.  The ATP molecule has five components.  A chemical called adenine also used in the cell as a DNA nucleotide (sub-unit of DNA) is attached to a sugar.  Together they are called adenosine.  The adenosine is connected then to three phosphate groups.  That is where the "Tri" in triphosphate comes from.  The ATP molecule is very reactive.  So much so that it willingly gives off energy in a reaction.  When it gives energy to a reaction one of the phosphate groups fall off.  This changes the ATP to ADP (adenosine diphosphate).  An enzyme is needed for this to take place, but it releases a great deal of energy each time.  This is the ultimate form of energy for the cell.  All cellular process require the use of ATP to drive most reactions.
ATP is produced in the mitochondria on a phospholipid membrane called the cristae.  When glucose is broken down through a process called cellular respiration, ADP is converted to ATP by sticking a phosphate back on to the ADP.  When the Phosphate group is added back to the ADP energy has to be added to make it stick.  That very same energy is given away (used) in a reaction when ATP is changed to ADP.  This is how the reaction is written:

ATP          ADP + P + Energy
ADP + P  + Energy         ATP

ATP is made when glucose is metabolized and broken down into carbon dioxide and water.  The bonds that made up the glucose are then used to place the phosphates back on ADP.