What is the function of AVDD,AVSS and Vref of ADC in Microcontoller?
I'm going to assume based on the names you are talking about Atmel AVRs.

"AVDD" and "AVSS" are basically power supply and ground for the analogue portions of the circuitry. These must be connected to "VDD" and "VSS", but they are brought out separately so that you can add a filter on to "AVDD" for noise reduction, and so that digital noise doesn't get coupled onto the analogue ground.

"AREF" is an analogue reference voltage which allows you to configure the upper range of the ADC. When specified by register bits, AREF is used instead of AVDD to provide the ADC reference. The ADC range will be between AGND and AREF allowing you to use the full dynamic range of the ADC even if the peak amplitude of your input signal is smaller than the supply voltage.