It is mainly number of cores, clock speed and amount of L3 cache and to a lesser degree memory speed.
In your case you are running an i7-930 with a core speed of 1603.7 MHz on the first 3 cores and 2940 MHz on the last core. BCLK 133.6 with 8 MB L3 cache and with 24 GB memory. I don't know what causes the different core speeds, but normally all cores would run at the same speed. Also your memory runs at 1066 speed.