This video is a very fast yet thorough outline of what the "no cloning" theorem entails. Cloning has been a current hot topic in genetics and biology for quite a while. However, it is also common knowledge that even though you can clone the DNA of a living being, there are other influencing factors that can change the properties of the cloned being, such as epigenetics, biochemistry, and so on.
Taking a look at quantum mechanics appears to be proof of principle that true cloning is impossible. When you have one of something, the laws of the universe imply that even with the same building blocks you cannot create a perfect copy. However, if you start with several of the same thing, then you can make more copies. Given that there is one of each of us and that we are all unique, we cannot clone anyone to create such a perfect copy that you cannot differentiate between the two. This simplified version of the no cloning theorem explains as much using mathematical proofs and universal laws.