Epigenetics refers to changes in traits that are not due to changes in DNA sequence. It is a growing field in biology, as we come to understand more and more how gene expression is not controlled purely at the sequence level.
There are many ways to alter gene expression without directly changing the DNA sequence being affected. For example, DNA is packaged into a DNA:RNA:protein complex called chromatin. Chromatin structure can be altered to change how tightly packed the complex is. The more tightly packed the chromatin, the more inaccessible the DNA is to the molecular machinery required for transcription, meaning gene expression is decreased. Very highly expressed genes will often be associated with very loosely packed, highly accessible chromatin.
A key feature of epigenetic changes is that they are heritable, with epigenetic marks being carried forward through cell divisions or even from one generation to the next. For example, research suggests that the epigenetic state of your DNA could be affected by your grandparents' lifestyle. Understanding how epigenetics contributes to trait variation adds yet another layer of complexity to what ultimately decides what phenotype an organism displays.
Extracting epigenetic information from cells
In the same way that we can learn genetic information via sequencing, there are several methods available to investigate the epigenetic state of DNA: