DNA methylation, referring to the reversible methylation of the 5 position of cytosine by methyltransferases, is a major epigenetic modification in multicellular organisms.  In mammals, this modification primarily occurs at CpG sites, which in turn tend to cluster in regions called CpG islands.  There is a small fraction of CpG islands that can overlap or be in close proximity to promoter regions of transcription start sites.  The modification may also occur at other sites, but methylation at either of these sites can repress gene expression by either interfering with the binding of transcription factors or modifying chromatin structure to a repressive state.
Disease condition studies have largely fueled the effort in understanding the role of DNA methylation.  Currently, the major research interest lies in investigating disease conditions such as cancer to identify regions of the DNA that has undergone extensive methylation changes.  The genes contained in these regions are of functional interest as they may offer a mechanistic explanation to the underlying genetic causes of a disease.  For instance, the abnormal methylation pattern of cancer cells was initially shown to be a mechanism through which tumor suppressor-like genes are silenced, although it was later observed that a much broader range of gene types are affected.