Critical Hits deal +100% extra damage (i.e. twice as much damage) by default. Critical Damage Bonuses can further modify this value.
Attacks usually use your weapon's base Critical Hit Chance, while Spells and some other skills have their base Critical Hit Chance listed on the skill.
Most modifiers to Critical Hit Chance are percentage based. For example, gaining 100% increased Critical Hit Chance on a base Critical Hit Chance of 7% would result in a final Critical Hit Chance of 14%.
Critical Damage Bonus
Multiplies the damage dealt by Critical Hits. Default value is +100% (i.e. twice as much damage).
Critical Weakness
Critical Weakness causes hits against affected targets to have +0.5% to Critical Hit Chance, and can stack up to 20 times. If not otherwise specified, each stack lasts for 4 seconds.
Inevitable Critical Hits
Hits which could potentially be a Critical Hit but do not roll a Critical Hit will re-roll Critical Hit chance until they succeed.
Hits have 30% less Critical Damage Bonus for each time Critical Hit chance was re-rolled.