why it's important for the body.
I think you are more interested about the results of the deficiency, right?
B12 deficiency:
Vitamin B12 deficiency can potentially cause severe and irreversible damage, especially to the brain and nervous system. At levels only slightly lower than normal, a range of symptoms such as fatigue, depression, and poor memory may be experienced.
Vitamin B12 deficiency can also cause symptoms of mania and psychosis.http://en.wikipedia.org/wiki/Vitamin_B12#DeficiencySymptoms of Vitamin B12 Deficiency
A deficiency of vitamin B12 can lead to vitamin B12 deficiency anemia. A mild deficiency may cause only mild, if any, symptoms. But if untreated, it may cause symptoms such as:
Weakness, tiredness, or light-headedness
Rapid heartbeat and breathing
Pale skin
Sore tongue
Easy bruising or bleeding, including bleeding gums
Stomach upset and weight loss
Diarrhea or constipation
If you think you have symptoms of vitamin B12 deficiency, you can ask your doctor for a blood test to check on that.http://www.webmd.com/food-recipes/guide/vitamin-b12-deficiency-symptoms-causesAre you by any chance a vegetarian or vegan? B12 is present to only to non-vegan foods like meat, eggs, milk, cheese, fish etc. Also, some companies started fortifying with B12 foods like cereals, but I don't know if that will be a good source of B12.
If you consume dairy/eggs then you shouldn't have any problem. For example, couple of glasses of milk are almost enough to cover the daily needs of B12. But if you are not consuming dairy, then I think that your best bet is to take supplements of vitamin B12, or to look more thoroughly about the fortified-with-B12 foods that are on the market.
On the other hand, if the deficiency is not the result of your diet, but the result of a disorder, then you must consult a doctor for some special treatment.