Great info on that page! I understand better now. In experimenting with DC power, I would connect a lamp's positive (+) side to COM and NO leading to battery's positive terminal. Then the lamp's negative (-) side to battery's negative terminal. So when does a wire get connected to "NC: normally closed relay interface"? Also, the backend of the relay would be connected to the arduino Vcc, GND, and digital output pin. It was mentioned that switching a relay off using DC power could melt/destroy it.
A DC spark / arc could continue for some time, acting like a miniature welder and welding relay contacts together.
Instead use a Power Field Effect Transistors. Since my relay says SRD-12VDC-SL-C then I should be safe using 12VDC devices, right?? 8)