ChrisHigs:
Ah, so it just gives a more accurate 8MHz clock than the built in one?Thanks
On the majority of Arduino boards, the crystal is 16mhz (thus allowing it to run at 16mhz instead of 8mhz). 3.3v/8mhz pro mini's may use an external 8mhz crystal.
Note that the small, 3-pin, surface mount "crystals" are actually ceramic resonators, which are not as accurate as crystals (but better than the internal oscillator)