1) A triac is a bilateral switching device that can conduct current in both directions between its main terminals, unlike an SCR which is unidirectional. It consists of two SCRs connected in inverse parallel. 2) A triac turns on when a small positive or negative gate current is applied, allowing current to flow between its main terminals in either direction. It can be used to control power in AC circuits through phase control. 3) Triacs find applications in light dimmers and motor speed controls. However, they can cause electromagnetic interference due to sudden current changes, producing harmonics that may interfere with other electronic devices. Filtering is required to reduce harmonics to acceptable levels.