clock source for microcontroller stk500

what's the difference between using the microcontroller clock vs. using an external crystal or osscillator? what's the difference between the oscillator and the crystal? do i need only 1 clock for the microcontroller? does it have an exact value? or should it just be approximately the same value?

i happen to be using an atmega16 microcontroller on an stk500 board; i'm currently using the board osscillator and i want to use the microcontroller clock instead... any hints? i'm very new to the topic!

thanks a million for the help!

hani a.