Taken from the Arreat Summit:
Open Wounds
This is a chance of making a monster bleed uncontrollably. They lose health while bleeding. Open Wounds Items stack in most cases.
Duration: 200 frames (that is 8 seconds).
The damage per frame seems to be the following (where Clvl is the attackers level, that is the player's level usually):
Clvl=1-15: (9*Clvl+31)/256
Clvl=16-30: (18*Clvl-104)/256
Clvl=31-45: (27*Clvl-374)/256
Clvl=46-60: (36*Clvl-779)/256
Clvl=61-99: (45*Clvl-1319)/256
If you prefer per second, just multiply by 25:
Clvl=1-15: 25*(9*Clvl+31)/256
Clvl=16-30: 25*(18*Clvl-104)/256
Clvl=31-45: 25*(27*Clvl-374)/256
Clvl=46-60: 25*(36*Clvl-779)/256
Clvl=61-99: 25*(45*Clvl-1319)/256
Some examples:
Clvl 10: 11.8 per sec over 8 seconds for a total of 94.5 damage. Clvl 30: 42.6 per sec over 8 seconds for a total of 340.6 damage. Clvl 50: 99.7 per sec over 8 seconds for a total of 797.7 damage. Clvl 70: 178.8 per sec over 8 seconds for a total of 1430.5 damage. Clvl 90: 266.7 per sec over 8 seconds for a total of 2133.6 damage.
Also, the damage is divided by 4 for a player target. In addition, for a missile versus a player target one should divide the damage by 8 instead.
Finally, versus bosses and champions (could be just bosses or special bosses) the damage is divided by 2.
It is possible to get up to 100% chance for Open Wounds. Anything above 100% is discarded.
There are a few things wrong with this info but it should give you an idea of what Open Wounds is.
To note: open wounds is not technically damage, it is negative life replenish, so it does damage but can not be resisted.
The damage listed on the summit is 1.10 info, from what I heard on an amazon basin thread (don't have a linky, if someone knows the thread Im talking about copy/paste info here please) there was a change done to open wounds in 1.11, damage done by open wounds is increased, however duration could be reduced by fade.
Still same concept, its negative replensih that is dependant on your level. If you want more open wounds damage level your character higher, damage increases every level.
Open wounds does not stack with multiple hits however retriggering OW will reset the timer so that will result in more damage.