I ordered the 8-relay version no too long ago. These boards are really nice, but it did confuse me at first too.
If you supply your arduino with 5v directly, just supply this board to the Vcc pin the same way. Otherwise like LarryD said, from the +5v on the arduino to the Vcc pin on the relayboard.
Connect Arduino GND to any of the relay GND (or connect relay GND to - on your 5v supply if you are using one)
It's just like you said, and like LarryD said. You can either connect everything to the arduino, or just the outputs like I did.
This is how I ended up hooking up my digispark to it: https://docs.google.com/drawings/d/1LRfwtz8sct8Hm08lqeQ6B93HI6pUo5tBVU7Jr4_pQUk/edit?usp=sharing
I put it upside down on the pins, to get the order right. That's why the outputs are crossed :P
Then just control the relays with high/low.
You don't need to put anything at the gnd/vcc/JDvcc. the GNDs are connected internally, so is the Vcc. The jumper is used so that Vcc is powering the Relays (JDVcc goes to the relays). They are 5v relays tho, so no need to use anything else than 5v.
So if you are using a 12v battery, I guess you should connect that to the Arduino VIN and then power the relay from the +5V pin on the arduino.