It's the accumulation of positive or negative charges on insulators or conductors. These charges can vary, based on the stored capacitance of certain items with respect to an object having a corresponding opposite charge. The word static simply means the charge cannot be equalized or transferred through electromotive force until there is a decrease in the capacitance between two objects. An example of this can be your hand (which may hold a negative charge) as it approaches a doorknob (which may hold a positive charge). As your hand moves closer to the doorknob, the capacitance between the object and your hand will decrease. This results in a flow of current between your hand and the doorknob. This transfer is known as electrostatic discharge, or ESD.