Critical hit damage
From Diablo Wiki
Critical Hit Damage (CD) determines how much damage a critical hit deals in Diablo 3. Raising this figure is a key way to increase a character's DPS, and skills and items that add to this stat are quite valuable.
The base critical hit damage of a brand new character is 50%, but well-equipped characters should deal at least 300% CD, and figures over 500% are not uncommon in very good gear (as of v1.06).
Critical hit Chance (CC) works in tandem with CD, and is as or more important. While Critical Damage is a huge factor in a character's overall damage output, Critical Chance is as well, plus many procs trigger off of crits, and a fairly high Critical Chance is considered essential for many builds to function properly.
- Refer to the Critical hit Chance article for more details.
 Critical hit Damage
CD can roll as an affix on the following item types, and on these types it may occur on magical items, rares, and in the random affixes on legendaries. (Many legendaries of the types have some inherent CD value, which will always appear and will not stack with a random CD affix.)