Epigenetics are chemical markers attached to our DNA that have ability to essentially turn genes on and off. DNA doesn't change over the course of our lifetime, and it's exactly the same in every cell in our bodies. This means that if you examined the genetic sequence in both your skin cells and your brain cells, you wouldn't be able to tell them apart.
But there are differences between those cells, because they perform very different functions. The difference is in the epigenes; these markers turn on the genes that tell a skin cell to behave like a skin cell, while turning off genes that it doesn't need, such as those that would make it act like a brain cell. In the beginning, each cell has the potential to grow into any type of tissue. This is how a single cell -- a zygote -- can form into all of the different types of tissues necessary to become a human being.
Epigenetics involves a process called DNA methylation, which takes place at a location along the DNA sequence called CpG islands. Our DNA comprises sequences of base pairs, abbreviated C, G, T and A. CpG islands are regions where the C, or cytosine nucleotide, is attached to its corresponding G base, or guanine nucleotide, by a phosphate. A methyl group is a "tag" attached to the C base. The genes are chemically altered, but the genetic sequence itself isn't changed.
The presence of the methyl group tells a gene not to express itself, also known as gene silencing. When the cell replicates, the methyl group will be passed down to the new cells. Three different enzymes, known as DNA methyltransferases (DNMTs), are responsible for DNA methylation. Epigenetics also determines how DNA in that particular genetic area will be wound around genetic proteins called histones. The more compact the DNA, the less likely it is for those particular genes to be expressed.