VCC / VDD

Vcc - Voltage Collector Collector
Vdd - Voltage Drain Drain
Vee - Voltage Emitter Emitter
Vss - Voltage Source Source

Vcc -- For TTL circuitry, this is the +5 volts which sits on the collector of the bi-polar transistor output circuit.
Vee -- For TTL circuitry, this is the GND which sits on the emitter of a bi-polar transistor output circuit.
Vdd -- For CMOS circuitry (like the PIC) this is the +5 (or +3) voltage that sits on the 'drain' of a
MOS-FET transistor output circuit.
Vss -- For CMOS circuitry, this is the GND that sits on the 'source' of a MOS-FET transistor output circuit.

Ну и ещё:
Positive voltages:
Vcc- Positive supply voltage of a Bipolar Junction Transistor.
Vdd- Positive supply voltage of A Field Effect Transistor

Negative voltages/ground:
Vee- Negative supply voltage of a Bipolar Junction Transistor.
Vss- Negative supply voltage of A Field Effect Transistor.

The letters c,d,e and s originated from the name of the legs of the transistors
Collector, Drain, Emitter and Source.

взято здесь , а от иногда спрашивают )