Almost all integrated circuits (ICs) have at least two pins which connect to the power rails of the circuit they are installed in. These are known as the IC's power supply pins. However, the labeling of the pins varies by IC family and manufacturer.

Typical supply pin labeling 
BJT FET       
VCC VDD V+ VS+ Positive supply voltage 
VEE VSS V− VS− Negative supply voltage 

The simplest labels are V+ and V−. However, internal design and historical tradition have led to a variety of other labels being used. Also, V+ and V− may be confused with the + and − voltage inputs to ICs like op amps.

Sometimes one of the power supply pins will be referred to as ground. In digital logic, this is nearly always the negative pin; in analog integrated circuits, it is most likely to be a pin intermediate in voltage between the most positive and most negative pins [citation needed].

Double subscript notation uses similar looking placeholders with subscripts. In that notation the subscripted letters denote two points.

History
VCC (note: lower case is often used instead of subscript, e.g. "Vcc") is an electronics designation that refers to voltage from a power supply connected to the "collector" terminal of a bipolar transistor. In an NPN bipolar junction transistor, it would be +VCC, while in a PNP transistor, it would be −VCC. There is debate over the origins of the double letter subscript naming convention. One proposal is that it originated as an abbreviation of the supply voltage for a common collector amplifier with the other power supply names mimicking this fashion. Double letters may also have been used to clearly indicate that a power supply voltage is being referred to.

In general, double letter subscripts refer to power supply voltages. For example, VEE refers to the "emitter" voltage as supplied by the power supply pin. In NMOS logic, VSS refers to the "source" voltage, and VDD is the "drain" voltage likewise.

Within a circuit, single letter subscripts refer to that voltage relative to ground; for example, VC is the "collector" voltage relative to ground. Two different letters indicate the voltage between two terminals; for example: VBE is the "base" to "emitter" voltage drop, while VCE is the "collector" to "emitter" voltage.

Modern use
CMOS ICs have generally borrowed the NMOS convention of VDD for positive and VSS for negative despite the fact that both positive and negative supply rails actually go to source terminals (positive supply goes to PMOS sources, negative supply to NMOS sources). ICs using bipolar transistors have VCC (positive) and VEE (negative) power supply pins.

In single supply systems (e.g., most modern digital and analog circuits) the negative power supply pin is also commonly referred to as GND. In "split rail" supply systems (e.g., older analog circuits) positive, negative and ground power supply pins are used.

More advanced chips will often have pins carrying voltage levels for more specialized functions in or out of the chip and these are generally labeled with some abbreviation of their purpose. For example VBUS for the 5 volt supply needed for a bus-powered USB device or Vref for the reference voltage for an analog-to-digital converter.

From: http://www.answers.com/topic/ic-power-supply-pin?cat=technology   ---------------------------------------------------------------------------------------------------------------------------------------------------- VCC：C=circuit 表示电路的意思, 即接入电路的电压； 
VDD：D=device 表示器件的意思, 即器件内部的工作电压； 
VSS：S=series 表示公共连接的意思，通常指电路公共接地端电压。
GND：在电路里常被定为电压参考基点。
VEE：负电压供电；场效应管的源极（S）
VPP：编程/擦除电压。

1、对于数字电路来说，VCC是电路的供电电压,VDD是芯片的工作电压（通常Vcc>Vdd），VSS是接地点。
2、有些IC既有VDD引脚又有VCC引脚，说明这种器件自身带有电压转换功能。
3、在场效应管（或COMS器件）中，VDD为漏极，VSS为源极，VDD和VSS指的是元件引脚，而不表示供电电压。
4、从电气意义上说，GND分为电源地和信号地。PG是 Power Ground（电源地）的缩写。另一个是 Signal Ground（信号地）。实际上它们可能是连在一起的（不一定是混在一起哦！）。两个名称，主要是便于对电路进行分析。 进一步说，还有因电路形式不同而必须区分的两种“地”：数字地，模拟地。
数字地和模拟地都有信号地、电源地两种情况。数字地和模拟地之间，某些电路可以直接连接，有些电路要用电抗器连接，有些电路不可连接。

Vcc 来源于集电极电源电压, Collector Voltage, 一般用于双极型晶体管, PNP 管时为负电源电压, 有时也标成 -Vcc, NPN 管时为正电压. 
Vdd 来源于漏极电源电压, Drain Voltage, 用于 MOS 晶体管电路, 一般指正电源. 因为很少单独用 PMOS 晶体管, 所以在 CMOS 电路中 Vdd 经常接在PMOS 管的源极上. 
Vss 源极电源电压, 在 CMOS 电路中指负电源, 在单电源时指零伏或接地. 
Vee 发射极电源电压, Emitter Voltage, 一般用于 ECL 电路的负电源电压. 
Vbb 基极电源电压, 用于双极晶体管的共基电路.

详解： 
在电子电路中，VCC是电路的供电电压, VDD是芯片的工作电压： 
VCC：C=circuit 表示电路的意思, 即接入电路的电压， D="device" 表示器件的意思, 即器件内部的工作电压，在普通的电子电路中，一般Vcc>Vdd ! 
VSS：S=series 表示公共连接的意思，也就是负极。 
有些IC 同时有VCC和VDD， 这种器件带有电压转换功能。 
在“场效应”即COMS元件中，VDD乃CMOS的漏极引脚，VSS乃CMOS的源极引脚， 这是元件引脚符号，它没有“VCC”的名称，你的问题包含3个符号，VCC / VDD /VSS， 这显然是电路符号。

我们在PCB设计时，会在画原理图时碰到对地的处理，比如：大地，模拟地，数字地，电源地等等，那么在设计时就要考虑，有些模拟地和数字地可以连在一起，有些就不能连在一起，大多数是相互独立的。