Solid-state relays are switching relays that do not require the use of any mechanical parts. This usually allows them to have a higher lifespan than a common electromechanical relay.  They are also most typically designed as a simple on-off switch with a terminal for power and terminal for load switched when an external control signal is passed to the relay through another terminal.  When this occurs the switching happens very quickly, and the load is given power, usually through the use of a power MOSFET transistor. 

 

Relays can be designed and used in either an AC or DC switching capacity, but the internals have to be modified to work for either scenario.  DC relays can operate with a single MOSFET, with the source and drain connected to the power and load of the main circuitry, and the control signal attached to the pass-gate.  They also can have multiple transistors aligned in parallel to allow for a higher current flow potential, which can be rated into the 100s of amperes.  AC switches require at least two transistors because one MOSFET cannot inhibit current in both directions when the relay is in an off state.  Two transistors with their sources connected are used to block current when off and then pass power when the control signal is enabled.