http://en.wikipedia.org/wiki/Sprite_(creature)This is describing sprites as creatures. However sprites can also be used to describe small white balls of energy. They can be present before,after, or during spells. They are energy that is manifesting. Imagine the center of them is a magnet that attracts more energy. That builds up into a ball. Just as you can easily combine two separate drops of water the same thing is possible with energy.
http://en.wikipedia.org/wiki/Sprite_(lightning)This is the scientific term using sprites as energy. It has to do with lightning but the idea here is that sprites are also energy.
If you are capable of working with the energy well enough you could potentially make it take any shape you want. But usually they are balls of energy and are best to not be messed with.
There is my two cents, anyone else want to contribute to the tip jar?