Methylated DNA immunoprecipitation (MeDIP or mDIP) is a technique used to study methylation (gene silencing) patterns in the genome. An antibody raised against 5 mthylcytosine is used to pick out the methylation. This is because methlyation occurs in CpG sites throughout the genome.
For a graphical representation of the technique, look here
Many cancer researchers use this technique to see which parts of the genome are silenced (via blockage of activation by methylation on the promoters and activation domains) in normal and diseased tissues.