VDDC is voltage to the core. They follow a table in the bios for a certain state at a certain power level, pre-defined according to TDP to match a clock state. This power level is called a VID, and is set from the factory in the bios according to chip binning (aka ASIC level measured in gpuz). VDDC going down just means that gpuz is measuring the actual voltage output from the VRMs to the core. What you set in Afterburner or Trixx is a VID. The loss you experience is "Vdroop".
VCore: The core supply voltage of an 'important' chip like your CPU or GPU, usually not the Northbridge. Most frequently used to indicate CPU voltage.
VDD: The supply voltage to your Northbridge chip or the supply voltage for the input buffers and core logic of your memory chips (mostly on graphic cards).
VDDQ: The supply voltage to the output buffers of a memory chip.
VTT: Tracking Termination Voltage. Compared to VREF to determine Hi/Lo
VMem: Supply voltage to a memory chip.
VDDR, VDimm: Supply voltage to the memory on your motherboard.
VRef: Reference voltage for the input lines of a chip that determines the voltage level at which the threshold between a logical 1 and a logical 0 occurs. Usually 1/2 VDDQ.
VGPU: The supply voltage to your graphic card's processor.
Terms used by ATI internally:
VDDC: GPU Voltage
MVDDC: Memory Core Logic Voltage
MVDDQ: Memory Voltage supplied to the output buffers of the video card memory.
VTT: Termination Tracking voltage for video card memory.
Electrical Engineering Information
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.
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.
The doubled suffix indicates that the voltage is "common", i.e. it is the supply voltage to one or more collectors (in the case of cc) and not just the voltage at a specific collector. Similarily, Vee is a common voltage for all emitters etc.