A primer app note(PDF) on silicon transient voltage suppressors by Microsemi.
Silicon transient voltage suppressors (TVSs) are clamping devices that limit voltage spikes by low impedance avalanche breakdown of a rugged silicon pn jucntion. They are used to protect sensitive components from electrical overstress such as that caused by induced lightning, inductive load switching and electrostatic discharge.
For more detail: App note: What is a silicon transient voltage suppressor and how does it work
