Shown on this slide is a Triac, illustrating what it is that and the basic operation. A Triac is an AC switch with PN/PN junctions, which can conduct current on both halves of the AC cycle as shown in the diagram. Therefore it is used as a true AC switch. A Triac has two main terminals, A1 and A2, which are where the current passes, these are the power terminals of the device.  Additionally there is a gate, which controls switching the Triac on, triggering it on during the conduction cycle. The main difference between a transistor and a Triac is that the Triac is normally triggered with a pulse of current into the gate terminal which latches it on for the half cycle that it is conducting in. A transistor does not typically latch on, if current is applied to the base to turn it on, and then removed the transistor will turn off immediately. A Triac will not turn off until it passes through the current zero crossing on the AC cycle.