

Critical Hit occurence  
To start out, a critical hit occurs when the critical hit value exceeds a number that is randomly generated for each hit. Naturally, the higher the value, the better the chance that it will be higher than that randomly generated number. As with many other variables in the game, the scale for critical hit value and the randomly generated number goes from 0 to 255.
In G/S/C all pokemon have a base critical hit value of 17, which gives a 6.6% chance of getting a critical hit. This value can be modified by the moves and/or items used by a pokemon. These increases will change the value to different levels on a fixed scale for critical hits. The scale is as follows:
All pokemon start at the base level of zero, therefore the only other thing to consider is the moves and items that can increase that level and by how much they do so. Those values can be shown as follows:
*Note: The use of these two items are the only Critical Hit modifiers that will function on the pokemon that are properly equipped with them. This is done to remove the possibility that the Critical Hit level will exceed 4. The information in the two tables above are all that is really needed to determine what the chances of a critical hit will be for a given attack. However, a couple of examples may clarify things further, in case the information above hasn't quite solidified in the reader's mind. Example 1: A Persian using the move 'Slash'.
Example 2:A Tyranitar with the Scope Lens item, using the move Earthquake.
Example 3:A Machamp, that has been Baton Passed to by a pokemon that used Focus Energy and Swords Dance once, is equipped with a Scope Lens and is using the move 'Cross Chop' on a Skarmory.

Critical Hit damage 
To put it simply, Critical Hits do twice the amount of damage of a non Critical Hit. Of course, it's not entirely that simple, or no one would put up a page about it.
In R/B/Y when a critical hit was scored, all stat modifications would be dropped and the damage would be calculated from that. This made it bad to combine certain moves on a pokemon, like Swords Dance and Slash. In addition, if a pokemon built up defenses with a stat modifier, such as Acid Armor, it would be completely ignored when a critical hit happened. In G/S/C the system has been changed and there are now other moves which may affect the damage that a move does. So, which of these modifiers will be included in the doubled damage calculation and which won't? It depends upon which category they fall into. 1.If the effects of a move or item affects the damage that is currently being performed (such as Light Screen or Rain Dance), it will be included in the damage calculation. This is regardless of whether the effects would increase or decrease the damage that the critical hit would cause. 2.If stat modifications are in effect on one or both of the pokemon, then two cases will be checked. One case will be with all of the stat modifications in place, the other will be with all of the stat modifications removed. Whichever of these two cases results in higher damage will determine whether all stat modifications are retained or all stat modifications are removed for the calculation of the damage of the critical hit. Basically, the only thing that may be dropped from the calculation when determining the damage for a critical hit are all of the stat modifications in play. It may not have gelled as to how this exactly works for stat modifications, so let me give a few examples to make sure it's solid (If it's already solid for you, stop wasting your time and go read something else, this is for the people who are still a litle bit unsure). Example 1: The attacking pokemon has used Curse 3 times. The defending pokemon has used Curse 2 times. At the time of the critical hit, the Attack will be 250% of the unmodified stat, while the Defense will be 200% of the unmodified stat. Keeping these stat modifications will result in more damage than if all stat modifications were removed, so all stat modifications are retained for the calculation. Example 2: The reverse of the previous example. The attacking pokemon has used Curse 2 times. The defending pokemon has used Curse 3 times. At the time of the critical hit, the Attack will be 200% of the unmodified stat, while the Defense will be 250% of the unmodified stat. Dropping these stat modifications will result in more damage than if all stat modifications were retained, so all stat modifications are dropped for the calculation. Example 3: The attacking pokemon has been hit by 2 Growls and has used Curse 3 times. The defending pokemon has been hit by Screech 2 times and has used Curse 6 times. At the time of the critical hit, the Attack will be 150% of the unmodified stat, while the Defense will be 200% of the unmodified stat. Dropping these stat modifications will result in more damage than if all stat modifications were retained, so all stat modifications are dropped for the calculation. 