Epigenetics is a term which is used to describe situations in which genes express themselves differently, although the underlying DNA is the same. For example, if a calico cat is cloned, the clone will not look physically identical, despite the fact that their genetic code is the same. This is because the expression of genes is influenced by epigenetics.
One could think of the genome as a book of blueprints, laying out a number of options in the form of genes. The epigenome is like the contractor who goes through the book, deciding which options to include in a house. Two different contractors can build radically different houses from the same book of blueprints, in the same way that two organisms with identical DNA can look very different.
Epigenetics plays an important role in the development of living organisms. When an egg and sperm cell first meet to form a zygote and start replicating, epigenetics comes into play, telling the cells what to develop into. Hair and skin cells, for example, have exactly the same DNA, but the DNA is expressed differently to create two different cell types. Epigenetics determines how the genes are going to be expressed. Because so much cell differentiation is going on during fetal development, the fetus is also very vulnerable to environmental factors which can influence epigenetics, like the ingestion of a toxin which switches a gene off or on.
A number of processes can be involved in epigenetics, including DNA methylation, in which structures attach to the DNA to turn specific genes off, and RNA interference. Genetic expression can be influenced by inheritance, as when two blue eyed people have a blue eyed child because their epigenomes cause the gene for blue eyes to be expressed and this pass this trait on, and it can also be influenced by environmental factors, because the environment can influence someone's epigenome.
Twin studies, in which researchers follow the lives of identical twins, can reveal interesting information about epigenetics, because they allow people to see how genetically identical organisms can diverge in response to environmental factors. Epigenetics researchers have also studied things like the impact of environmental toxins on pregnancy, and the history of epigenetic change in various families and populations.
Changes in the epigenome can cause the development of cancers and other unusual growths. Some research also suggests that these changes could potentially be reversible, which could open up a number of new medical treatments; while the already expressed genes couldn't be changed, treatment could be used to prevent further expression of a harmful gene.