How are epigenetics involved in regulating gene expression?

Epigenetic control of gene expression involves mechanisms that do not alter the sequence of DNA itself. External factors acting upon cells translate their signals via cascades to the nucleus, where they act to modify the gross structure of DNA and thus impact upon transcription. In this way, environmental factors can be translated into direct, heritable genetic changes. Epigenetics help control differential gene expression in separate tissues within the body, and also allow for the environment to impact upon gene expression.
Epigenetic modification works by modifying the transcription machinery's ability to access genes encoded on the DNA double helix. Several types of epigenetic chemical modification work to alter gene expression. DNA methylation is one of these, which has an inhibitory effect on transcription of the target region of DNA by decreasing its availability to transcription machinery. Modification of the histones - proteins around which DNA is coiled - also plays a crucial role in epigenetics. Alterations to the histones (including acetylation and methylation) can have a number of different upregulatory and inhibitory effects on gene expression.