These notations are used in describing voltages at various common power supply terminals (at these points, only a wire lead exists between the point and a power source) of a given circuit. It turns out that these common voltage terms map to transistor technology as follows:
BJT FET "Vxx" meaning
Vcc Vdd Positive supply voltage
Vee Vss Negative supply, ground
http://encyclobeamia.solarbotics.net/articles/vxx.html