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:
Apparently this terminology originated in some way from the terminals of each type of transistor , and their common connections in logic circuits ( i.e. , Vcc is often applied to BJT collectors , Vee to BJTemitters , Vdd to FET drains , and Vss to FET sources ) . This notation then carries across to integrated circuits - TTL ICs were originally based on BJT technology , and so often use the Vcc / Vee terminology ; CMOS ICs are based on FET technology , and so often use the Vdd / Vss terminology .
The absolute distinctions between these common supply terms has since been blurred by the interchangeable application of TTL and CMOS logic families . Most CMOS ( 74HC / AC , etc. ) IC data sheets now use Vcc and Gnd to designate the positive and negative supply pins .
source: http://encyclobeamia.solarbotics.net/articles/vxx.html