A J valve is almost identical to a K valve but is equipped with a lever on the hand wheel allowing for a reserve pressure. It was used before submersible pressure gauges become easily available. Today J valves are not in use for recreational and technical diving anymore and limited to very specific circumstances by military, commercial or other specialized diving.
The K valve is the most common scuba cylinder valve. It has one knob on the side perpendicular to the post. Its clockwise rotation close the valve. K valves have either a DIN or a Yoke connector for the regulator.
A burst disc is a safety feature of cylinders valves to help prevent overpressurization. It will as the name implies burst with the excess pressure and drain the cylinder until it is at ambient pressure.