HOW IT WORKS:
Observing carefully the transistor inverter above - you will notice that there are 2 inputs to the inverter and 2 outputs. The electricity is always on to the +Vcc (the collector). Applying electricity to Vin (base) will control the transistor and ultimately control the value resulting at Vout. With a value of "off" (no electricity) at Vin - the transistor's gate is open and electricity is unable to pass through it to the emitter and toward ground. This means electricity is free to flow out the Vout.

When a value is placed on Vin then the transistor allows the electricity to flow from +Vcc and to ground. This will produce a value of "off" at Vout because electricity will always flow to ground (instead of somewhere else) if it is given a choice. (This is based on the principle: electricity will always choose the path of least resistance).