This a longish, review-type article that appears well-balanced. It is a very new field with lots of implications for humans. However, epigenetics in mammals in general and humans in particular is controversial.
As with so much in science, this story owes a lot to mice. The tale begins with a pregnant mouse in a laboratory in Boston, Massachusetts. Such is the unfortunate lot of a lab rodent that she was kept on a near-starvation diet when she was close to giving birth. As scientists expected, her babies were born smaller than usual. When they were raised normally, they later developed diabetes.
Now comes the twist. Even though these mice were well fed, their own young were also born unusually small and with a higher risk of diabetes. This was strange, because nothing had changed genetically and they hadn't suffered any problems in the womb or after they were born. They should have been perfectly healthy.
There are many definitions of epigenetics, but simply put, says Professor Marcus Pembrey, a geneticist at University College London and the University of Bristol, it is a change in our genetic activity without changing our genetic code. It is a process that happens throughout our lives and is normal to development. Chemical tags get attached to our genetic code, like bookmarks in the pages of a book, signalling to our bodies which genes to ignore and which to use.