From the Data Sheet:
AVCC is the supply voltage pin for the A/D Converter, PC3:0, and ADC7:6. It should be externally connected to VCC, even if the ADC is not used. If the ADC is used, it should be connected to VCC through a low-pass filter. Note that PC6..4 use digital supply voltage, VCC.
AREF is the analog reference pin for the A/D Converter.
If you're not using the A/D, you don't need to connect AREF.