Within the field of epigenetics changes in how the genes are expressed is studied, and if a particular gene is active or not. The so-called phenotype, that is, how an individual looks or behaves, is determined by whether a gene is expressed or not.
Epigenetic changes in the genome depend on different modifications in the DNA. Many diseases are believed to be influenced by epigenetic changes, including cancer, diabetes and schizophrenia.
Usually, epigenetic changes are not inherited to the next generation because the changes will be "erased" during the formation of the germ cells.