For several reasons, "identical twins" is a commonly used term but is not part of the professional medical language. Though identical twins have identical DNA they remain uniquely different from each other.
Identical twins, formed when one fertilized egg splits, are the only people in the world with identical DNA. Fraternal twins, on the other hand, are formed when two different eggs are fertilized. Genetically speaking, fraternal twins are no closer than normal siblings, sharing only about 50% of their genes.
At issue here is the long-standing debate of nature versus nurture which speaks to the total development of people. Adherents of the nurture position believe that at birth humans are essentially a blank slate, and that their environment as they grow and develop is the only factor that determines characteristics of the individual. Thus matters of choice of profession, mate, musical preferences, morality, etc. are determined by society. Believers in the nature position, on the other hand, say that genetics is crucial in development, and that the characteristics of an individual are determined at the moment of conception.
The data is fairly clear that both genetics and environment are approximately equally important in the development of an individual.
Several studies that lead to this conclusion involve identical twins who were separated at birth. Such twins have sometimes been raised in very different social environments. Nonetheless, there are often strong correlations between the later behavior of such twins: if one has become a teacher then the other has also become a teacher. Other characteristics that twins tend to share, even if raised in very different environments, include emotional characteristics of their choice of mate, preferences in music, and more.
It is important to note that these correlations are not 100%. Just because one twin is a teacher does not guarantee that the other is too. Similarly, if one twin really likes the music of Bach does not guarantee that the other twin will also prefer that type of "music." Yet, these correlations are sufficiently strong that it is almost certain that they did not arise by pure chance.
Although identical twins have the same genotype, or DNA, they have different phenotypes, meaning that the same DNA is expressed in different ways.
Traits determined by phenotype, such as fingerprints and physical appearance, are the result of the interaction of the individual's genes and the developmental environment in the uterus. Thus, a DNA test can't determine the difference between identical twins, while a simple fingerprint can.
There are slight physical differences, such as one twin having a mole that the other does not. More dramatically, some identical twins are "mirror images" of each other, even though they have the same DNA. In such cases, which are rare, one twin's heart is on their left side (normal) and one twin's heart is on their right side (mirror image).
Not only are identical twins different in their physical makeup, but their life has unfolded with different experiences making them more diverse. Their body along with what they have experienced has shaped their mind, how they think, and the emotions they feel.