The chance of a critical hit depends on where're you aiming:
http://www.fonline-reloaded.net/wiki/index.php?title=Aimed_ShotLet's say you made a critical hit, the game will roll a d100 dice (a dice with 100 faces - in pen'n'paper we use 2x d10, that's two ten sided dice) to determine the effect of the critical hit. I don't suppose the server has some sort of analog, dice-rolling machine, so it's just a (pseudo-)random number from 0 to 99 (or 1 to 100?). Let's just call it a "roll".
If you have "Better Criticals" you add +20 to that roll, +25 if you have "HtH Criticals" (but I don't think they stack) and/or +10 if it was a "Silent Death" critical any "+Critical Power" mods of your weapon. "-Critical Power" mods of your targets armor and perks like "Man of Steel" that he/she has will lower the roll.
If after all the mods your role is:
0-20 - look at "Roll 0" in the tables
21-40 - look at "Roll 1" in the tables
41-60 - look at "Roll 2" in the tables
61-80 - look at "Roll 3" in the tables
81-100- look at "Roll 4" in the tables
101+ look at "Roll 5" in the tables
So let's say we were aiming for the head, and rolled 85 - there we have "x3, Bypass armor, Knockdown, Knockout (LK+2)". The hit target will be hit for 3x damage, his armor will be ignored (DT=0, DR=0%), he will be knockdown and if he fails a Luck +2 test he will be knocked-out.
What's a Luck+2 test? Well, you take a random number from 1 to 10 and if it's higher then the targets Luck+2, then the test is failed.
If I got something wrong, do correct me.