Does a pin act as a ground when "LOW"?

Im just wondering does a pin on an arduino act like a ground when its set to low? Using a NPN transistor the gate opens if i connect the base to a ground pin or set one of the digital pins to “LOW”.

Yes, low is approximately ground, or approximately zero volts. An output pin can "sink" up to 40mA.

Can safely sink 20ma.

The way to think of an output is to consider connecting to ground and connecting to 5V. In practice these two voltages will be 0.5V and 4.5V or so depending on the ammount of current flowing out of the pin - this is called sourcing, or flowing into the pin - this is called sinking.

h4344: Using a NPN transistor the gate opens if i connect the base to a ground pin or set one of the digital pins to "LOW".

Eh? If you connect the base of an NPN to ground, it will STOP it conducting. Were you thinking of a PNP transistor?

Open is ambiguous - open-circuit means off, disconnected, but an open tap is on, connected.

Try to always say "on" or "off" for transistors and other switching devices, "open" and "closed" are interpreted differently according to native tongue and context (an open switch is off, an open valve is on, for instance, but if you speak English as a second language this distinctiion is not obvious!

The reason for this is the original switches were knife-blade switches where you open the gap between conductors, or closed the gap. An open gap is no current flow, closed is current flow. In a valve for liquids open means the pipe is open (liquid can flow).

So "on" and "off" are better words to use here with an international audience.

MarkT: Open is ambiguous - open-circuit means off, disconnected, but an open tap is on, connected.

Try to always say "on" or "off" for transistors and other switching devices, "open" and "closed" are interpreted differently according to native tongue and context (an open switch is off, an open valve is on, for instance, but if you speak English as a second language this distinctiion is not obvious!

The reason for this is the original switches were knife-blade switches where you open the gap between conductors, or closed the gap. An open gap is no current flow, closed is current flow. In a valve for liquids open means the pipe is open (liquid can flow).

So "on" and "off" are better words to use here with an international audience.

Aha! Good catch. The OP may not have been confused at all. I read an open gate as allowing flow. Whereas he may have meant going open circuit.