Utilizing ATMega chip without Arduino board

After programming the microcontroller on my Arduino, I would like to remove it and put it into the board in my project. Is there anything I need to know about hooking a non-Arduino circuit up to the ATMega chip so that it will work the same (As when it's on the Arduino board)? I noticed that there are a few pins on the chip that I'm not sure what to do with, like the AREF and AVCC pins. Also, do the two pins called GND both go to ground, or is only one required?

Thanks

Here is one example:

http://gammon.com.au/forum/?id=11109

Both GND pins should be grounded. They keep the analog ground away from the digital ground to keep noise down, but you should ground both of them. For a similar reason, the AVCC pin should go to +5V. Both VCC and AVCC should have a 0.1uF decoupling capacitor (to GND). AREF can be left not connected.

You will need a resonator or crystal unless you are going to configure the processor to use the internal clock.