Believe me, I've been asked this question so much since my twins were born I've gotten this one down by heart...
Basically, twins fall into two groups:
-- Identical (sometimes called Paternal or Maternal Twins) They're created from combining one egg and one sperm. After a few days the one embryo will divide and create two identical babies.
-- Fraternal (sometimes referred to as not-idential twins) are created from two eggs that get fertilized with two seperate sperm. Basically they are no more alike than any other brothers and sisters.
My girls fall into this category. It took me about three hours after their birth to tell them apart. They had, at that time, the same hair and eye color (but all caucasion babies have blue eyes, I'm told). I think that the first three hours I was just 'getting over' a very intense labor / deliver with pain medication!!
I remember that when I was pregnant with my girls that I had read a 'theory' about a third set of twins. Basically it was described in a book that I had, simply titled "Twins".... This type of twins were referred to as 'polar body' twins. In the case of polar body twins, there is one single egg that seperates (or divides into two seperate eggs) before it is united with sperm. If both of the newly created eggs recieve sperm, then twins are carried. From what I read back then, the twins often can look even more alike than fraternal twins, often to the point of people thinking that they're identical. I guess that's because they share 50% of the same chromosomes, given to them by the mother's side of the family. I can see that if this were the case how they can look more alike; however, if one baby recieves, for example, dark hair and eyes from the father's side, then it's possible that 'twin B' would look very different than "twin A" if the first twin had recieved genes that was to create a blonde haired / blue eyed baby!
My girls, again fraternal, did look quite a bit alike for the first couple of years. Other people didn't notice it as much as me.... It's funny now though because as I look back at those 14 year old baby pics I'm really glad I wrote on the back 'who was who' because now they seemed to have look very much alike for the first few months!!
Now, however, the girls are a blonde and a dark brunette, both have blue eyes. The blonde haired one is often mistaken as the twin of her older sister (she's the same height / weight / and facial & body features are amazingly similar)... But I guess it's impossible to be her twin unless she just layed dormant for almost four years in my uterus!!