Some PLCs can have relay cards in the chassis that act like component electromechanical relays you would mount elsewhere in a control panel. You can bring one voltage source, say, 120V, to one side of the relay and the PLC will switch only that voltage to the relay's "other side" while bringing a second voltage, maybe 24V, to a second relay on the same card (watch your wire color). Other cards are solid state devices that take one voltage and distribute the voltage to each output on the same card when the program triggers the associated output.
The "internal" relays or "coils" are bits that signify a condition as being true or false.
Hope this helps.
Rube