I think terryking228 answered most of it.
The optos are the small rectangular black blocks with four pins.
The board in the other link doesn't have them.
JD-VCC is the 24volt relay coil supply and VCC is the 5volt Arduino supply.
Could be unhealthy for the Arduino to mix them up.
Relay ground is connected to relay coil ground (see diagram).
If you share that with the Arduino, then you loose the opto isolation.
Ground is not needed to drive the opto LEDs, because they are powered/connected between VCC and relay inputs.
Opto isolation is wise if you use the relays for 240volt mains.
The boards are not specified for mains power use, and opto isolation gives a second layer of protection
Opto isolation means the 24volt supply has to be used exclusively for the relay board(s).
Leo..