NO - Normally Open - when the relay is not energized or activated, this terminal is open-circuit.
NC - Normally Closed - when the relay is not energized or activated, this terminal is closed circuit.
COM - The Common Center Terminal in relays. This is the center terminal that closes circuit with NC-NO.
The terminals are potential free and floating and are insulated from the control signals or coil windings by many kilo volts. Many provide visible isolation. These terms apply to SSR or Big Contacters too.
Solid State Relays - SSR
They are called SSR as there are made of semiconductors and have no moving parts to wear out. They do generate heat depending on load and switching. Thyristors are used in most as they are rugged, mosfets and IGBT may also be used for various applications.
In this section you will find SSR designs and PCB layouts which you can study. These were made and sold by me years ago. So the PCB layouts have gone thru some revisions and they worked quite well.
Many machines and equipment use some SSRs in their products which may need replacement. So the SSR is not only sold to OEMs but also to Service-Maintenance Market. These are also modified as Output Modules for digital outputs coming from embedded systems or Computers.