A solid state relay (SSR) is just what it sounds like; an IC that acts like a mechanical relay. They allow you to control high-voltage AC loads from lower voltage DC control circuitry. Solid state relays, however, have several advantages over mechanical relays. One such advantage is that they can be switched by a much lower voltage and at a much lower current than most mechanical relays. Also, because there's no moving contacts, solid state relays can be switched much faster and for much longer periods without wearing out. Project description, more information and ordering can be found at https://www.sparkfun.com/products/10684 Datasheet: http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Components/General%20IC/S108,208T02%20Series.pdf