Cell Biology/Energy supply/Important chemicals
Time for a lesson in terminology. The methods by which a cell supplies and stores energy are quite complex, and there are many long, scary words involved, as well as many short, scary acronyms for them. Many students will look at a system such as the Krebs cycle and tell themselves "There are way too many confusing words here, this is not for me!" In reality, the basic ideas are not that complex, and once you familiarize yourself with several of the chemicals integral to the energy storing processes, both photosynthesis and respiration will stop being so scary.
ADP and ATP
Adenosine triphosphate (ATP), considered the universal energy currency of the cell, is a molecule with three phosphate groups attached to it. Though all three bonds contain stored chemical potential energy, the bond between the second and third phosphate groups are relatively easier to break apart, thus providing a means of energy storage for the cell. As a result, breaking this bond to release energy, and forming it again to store energy, is relatively easy for a cell to carry out.
When an ATP molecule is lacking it's third phosphate group, it is called adenosine diphospate (ADP). Here is a handy linguistic trick to remember which is which: a molecule of ATP has tied-in energy, but a molecule of ADP is drained of energy. If that doesn't work, just remember that chemical bonds store energy, so the molecule with another phosphate group ('tri-' (three) is one more than 'di-' (two)) bound to it will have more energy. Those are two strategies for differing the two.
The most important aspect of ATP is that it is used in all organisms as a universal currency of storing and transporting energy for immediate use. ATP is simple and easy to form, as well as easily portable throughout cell and tissue alike.