i7 950 - 4 cores, hyperthrading (8 threads), 3.06GHz clock speed (3.33GHz max TurboBoost frequency).
i7 875k - 4 cores, hyperthreading (8 threads), 2.93GHz clock speed (3.6GHz max TurboBoost frequency).
Google is your friend if you want more specific details, it's not practical to list them all here when they are better detailed elsewhere.
Aside from the differences in clock speed, I'd say the biggest difference is that the 875k has an unlocked multiplier, similar to be Black Edition AMD chips. This means it will be easier to overclock the CPU - you just need to increase the multiplier.