wexler By convention Vcc means the positive logic supply rail (*) 5V is a voltage, not a signal or supply name. Many logic devices have Vcc = 5V, many do not. Vin is as documented in the description of the board the input to the on-board regulator. So you can treat Vcc or Vdd as meaning "logic supply" as opposed to analog or motor supply, for instance. (*) Originally Vcc = "voltage collector collector", Vee = "voltage emitter emitter", Vss = "voltage source source", Vdd = "voltage drain drain", were used for various TTL and pMOS and nMOS families. Eventually as a great variety of devices slimmed down to just TTL and CMOS 5V, Vcc was used commonly for all logic (although it makes no sense for CMOS where both rails are FET sources!). Vdd is also used. I hope, this extended answer, helps