In this thread I'll be explaining what's known about the Black Ops 2 Zombies ranking system. I will update the thread with new information once I know about it. Please do reply or send me a message if you think any of this information is incorrect or if there is something you think should be added.
Players have to ability to rank up during the Zombie mode of Black Ops 2. You increase your rank by performing well overall and maintaining a high Kill / Death Ratio. Your KDR has a great impact on your ranking.
Every player starts with the same rank and can rank up 4 times.
Rank 1: Wooden Shield with 1 Bone
Rank 2: Wooden Shield with 2 Bones Crossing
Rank 3: Wooden Shield with Skull
Rank 4: Metal Shield with Skull + Knife
Rank 5: Metal Shield with Skull + 2 Guns
You can see how close you are from entering the next rank by the tally marks on the top left of your ranking. Your rank starts with no tally marks and goes all the way up to 5 tally marks, at this stage you will also gain the blue eyes with the emblems that contain the skull (rank 3,4 and 5). After 5 tally marks you will jump to the next rank.
Performance: As mentioned above, your KDR has a great impact on your ranking. When your KDR goes down and / or you perform below your average, your rank will go down. How much it goes down by depends entirely on how poorly you performed. Don't look surprised when you lose a few tally marks at a time.
Time: When you don't play Zombies mode for a 24 period you will start losing your ranking. The rate at which you lose your ranking is 1 tally mark per 24 hour period. It is possible to lose your main ranking & tally marks.
For example: Say your current rank is the Shield with Skull and 4 tally marks. If you go don't play the Zombies mode for 24 hours, you will drop down to Shield with Skull and 3 tally marks. If you don't play for another 4 days, you will only drop down to the Skull with Shield and 0 tally marks. You won't drop down to the Shield with the Crossing Bones.
The ranking system is updated at 12 AM GMT (7 PM EST). This is the exact time you will lose a tally mark if you haven't played for 24 hours.
Updated [12/20/12]: You CAN loose your main rank if you loose all your tally marks. (Thanks Jake)
Official Thread: Black Ops 2 Zombies Ranking System Detailed