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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
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 BJT emitters, 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.
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