DNA is normally stored all coiled up. There's a whole bunch of things that latch on to DNA that open up certain bits of the coils or tighten up the coils making them less accessible to the cell machinery, which in turn can turn genes on or off. All this extra control is called the Epigenome.
"Epi-: Prefix taken from the Greek that means "on, upon, at, by, near, over, on top of, toward, against, among."Which is basically all the things that attach to the chomrosomes that also have a roll in controlling the cell (these can be changed by environmental factors which has led some to believe (and rattle on about) Darwin was wrong and the theory of evolution is wrong, which is, in short, bollocks). Funnily enough, this extra layer of epigenetic control increases the amount of information involved in controlling the cell significantly. Which is brilliant for those of us who like disappearing into a morass of complex control networks. Even better, there's an initiative in Europe that's about to start mapping the human epigenome. More data, more stuff to figure out. <grin>