There are two types of valve commonly used with Co2
pin valve - requires the pin to be depressed by a small nipple in the female connection on your marker or fill station. Closes with the pressure of Co2 behind it
On/off - allows you to close the valve prior to disconnecting the bottle so you dont have Co2 spraying out like you do before the pin valve closes.
go for the on/off if possible as its less messy, doesn't damage your bottle o rings as much, and gives you a better seal when closed,
JJ