I believe that epigenetics is the next frontier in the study of genetics, and biology. Wikipedia's definition of epigenetics is a good start, if you're new to the field.
But I'd like to clarify something that might otherwise be confusing people about this new science. The scientific definition of "epigenetics" can vary depending on which aspect the geneticist focuses: Embryonic Development, or Inheritable Traits. (I'm simplifying this, but it's a good start.)
Most generally, epigenetics is defined as the process of determining traits in offspring, in ways that are above and beyond how traits are determined by genes. Even when a set of genes in an organism don't change, the way that those genes express can vary depending on the environment, and chance.
There at least two different areas of study in epigenetics (when we define it loosely). Perhaps the most confusion in understanding what epigenetics is about is in how these two areas of study overlap at the bio-chemical level, yet are still (currently) treated as two different areas of study.
1) Before epigenetics was discovered, there was (and still is) a field called Developmental Embryology (or just Embryology). When egg and sperm form a new organism's first cell, it is a pluripotent stemcell with genes in DNA in its nucleus, containing all the information that it needs to become a complex organism of many different types of cells.
2) Given two embryos with the same copy of genes (e.g. twins or clones), it is possible probable for one of the twins/clones to epigenetically express and develop differently from the other. But what's more interesting is how this happens, and how it's possible or even probable for a parent's environment and/or embryo's environment to affect how each of those sets of identical genes can express themselves differently, even in future generations of offspring. This area of study is almost always called Epigenetics.
Perhaps the study of molecular mechanisms that work to differentiate stemcells into various kinds of cells during embryonic development will remain a science considered solely in the realm of developmental biology, and not taught in the realm of "epigenetics". But my point here is that it will always remain important to understand that 1) the molecular mechanisms of gene expression (and therefore cell differentiation) during development are exactly the same kinds of mechanisms that can 2) cause epigenetic, heritable differences in offspring, even when they carry the exact same sets of genes.
(I'm putting this out there as a blog post, in case anyone can help me clarify the definition and significance of this new field of epigenetics. Any/all corrections appreciated!)