Critical hit affects weighting of the random number selection for attacks. For example if your DC is 100-300 and your critical hit is 0 you will hit completely random between 100 and 300, average hit will be 200 damage. If you have 100 critical hit then you will hit 300 damage 100% of the time. If your critical hit is somewhere between 0 and 100 then it will be affected in the same way but along a sliding scale. With 50 critical hit and DC 100-300 you will find your average hit will be about 250 damage.
Critical strike is a chance of hitting extra damage. For example if you have 6% critical strike then every time you hit there is a 6% chance of the hit being critical, it will do much more damage (I think approx 50% more than your usual max, but can vary, not tested this sufficiently to give fully accurate answer)