There are two kinds of twins. One kind has the same set of DNA. These are called “identical twins,” even though they might not be quite identical. These twins started out as one fertilized egg, but the embryo split in two early on when the embryo only had about 16 cells.

The other type of twins are called “fraternal twins.” They are as genetically alike as any other pair of siblings (brothers and/or sisters). They started out as two separate fertilized eggs.

We expect identical twins to be pretty similar and their looks and behavior, but they will have differences. That’s because our environment shapes us too. Our environment starts before we’re even born. People can also make a lot of decisions that affect how they look and behave. One twin might even choose to make those differences great. For example, if one twin likes sports and short hair, the other twin might focus on art and grow their hair long because they want to be seen as an individual, not as part of a pair of twins.

Scientists find it very useful to study identical twins that were raised apart or compare identical twins with fraternal twins to try to figure out which of our characteristics are determined more by genetics and which are determined more by environment. Which traits do you think fall into each of those categories?

I first want to make clear that there are two types of twins, identical and fraternal.

Detailed discussions of the two and their differences can be found on ScienceLine already. For this question, which seems to assume identical twins, know that at birth identical twins have the same DNA while fraternal twins have only half of their DNA in common. For fraternal twins, the answer to this question is relatively straightforward:
differences in appearance are due primarily to the differences in DNA. Fraternal twins are as similar to each other as any other set of siblings with similar variance in their traits ( phenotype ). For identical twins, other factors must be dominating. Notice the specification that the DNA of identical twins is the same only at birth (and even then, it is probably perfectly identical). DNA is not as static as is sometimes implied; our DNA and the parts of it that we use can change as we age.

Differences in environment and life experiences also contribute to changes in appearance.

If one twin goes outside and gets more sun than the other, the pair may have some difference in skin color and aging; differences in diet will affect height and weight; illnesses, exercise habits, and a plethora of other factors also contribute. I'd also like to point out that these effects of environment do not change the appearances only of identical twins; they happen to everyone, but identical siblings are the only group which gives us a "baseline" for comparison.

I am not sure what you mean by 'so different'. Twins are children by the same parents, but there is no reason why they have to be anything more than that. There are identical twins, who are clones of each-other, but if not identical, then there is no reason why twins would be any more similar than two non-twin children of the same parents.

Some twins happen in their mother when one embryo divides into 2 identical embryos, and identical twins are born.

Some twins happen in their mother when 2 of the mother's eggs are fertilized with 2 different sperm, and 'fraternal' twins are born. They look like siblings of each other, but they're not identical.

That is a good question! In fact, there are actually two “types” of twins!

Monozygotic twins are sometimes called “identical twins,” and they are born when a single egg is fertilized by a single sperm to form a single zygote, which divides into two embryos. This means that they share 100% of the same DNA, and as a result look almost identical.

On the other hand, dizygotic twins, which are sometimes called “fraternal twins” are born when two eggs are fertilized by two different sperm. This means that they do not share 100% of the same DNA, and that is why they can look different from each other.