The Relay is a mechanical switch. A tiny arm moves inside the relay providing either an open or closed contact. A FET switch (Field Effect Transistor) is a solid state switch, no moving parts. The FET switch is designed for interfacing to a PLC.
The maximum rating for the FET is 36 VDC @ 100mA. The Relay is designed for interfacing directly to a device such as a light buzzer or another relay. The maximum rating for the Relay is 60 VDC/VAC @ 1A.