I'm a mechatronics engineering student, and I know that electronics is a requirement in most engineering programs. I'm been dealing with electronics recently and I've been studying electric circuits. I've been noticing that in some of them instead of being closed (in diagrams) they are like this. My problem doesn't deal with the transistors, yet it deals that in some circuits I've found that they write something like in this case +9 V and 0 V. Also in some circuits I've found this: Could someone explain me please what does this GND stuff mean? I'm so confused. My confusion originates that in the fact that the circuit has a V+ and a - but what does the VGND have to do there? Thanks.