Critical rate = Chance your attacks will be "critical" and do more damage
Critical damage = Increases the damage your attacks will do if they are critical. For example, without any additional critical damage (e.g. From skills, gloves), say your critical hits do 200% normal skills. If you upgrade the skills and get gear that increase your critical damage, you can do more, like 250% damage instead of 200% on critical hits.
Easily, critical rate is the percentage of your damage becoming 120~150% randomly and Crit damage adds your damage percentages.
For example, if your critical damage of 60% means your damage of critical hits will randomly be 180%~210%.