Electrostatic discharge is the transfer of electrons from one object to another.
You probably know ESD better as static electricity, or static shock. You experience ESD far more than you might think. In essence, ESD occurs continually when any two or more objects come into contact. So, when you work on the internal components of your computer, you may be damaging or destroying the components in the computer via ESD, and not even feel it. Many computer shops sell special wrist-bands which aid in grounding users while working on computers.