StealthRT:
The relay board is powered by an external power supply. However, i DID have to attached the ground to the arduino also because when i did not, they relays did not come on after 5v was sent from the pins. After hooking up the arduinos' ground also, it worked.
There should be two separate "ground" connections on the relay board: one for the negative side of the power a supply that you use to power the relay board, and one for the negative side of the opto isolator inputs - only this one needs to be connected to Arduino ground.