Vitamin B12 has the most difficult and complex absorption path of any vitamin.
Basically it must be bound to a protein in the stomach in an acidic environment. Then this complex must travel down to the end of the small intestine (illium) where it is absorbed. So if one takes an antacid, or does not make the protein or enough acid or has an illness like Crohn's disease or has the ilium removed you will not absorb this.
http://en.wikipedia.org/wiki/Vitamin_B12
Spoiler:
See above for description and pictures.
If you lack enough vitamin B12, you will likely develop anemia and then neurologic damage. Plenty of folic acid (which they are giving now because it prevents Spina Bifida) will prevent the anemia from developing, so people get the neurologic damage and the cause gets missed.
Missing the diagnosis is tragic because if caught early enough the neurologic damage can be reversed.
Blood levels for vitamin B12 are controversial. Technically "normal" is any level > 200. However, 10% of people can develop neurologic damage with levels between 200 and 400.