What do you think the E represents?
Haha, ok wanted you to not cheat by reading on first.
Well, it could be EARNINGS, or it could be EMOTIONS. Over a long period of time, share prices will reflect the underlying performance of a company, whether it is creating value for shareholders by making a decent return on its investments.
However, over the very short term, it really is dictated by Emotions. Whether the prevailing mood in the room is of fear, or greed. Not mentioned much, but actually anger or fustration could play a part at times too.