The DMZ or Demilitarized Zone is the 'no man's land' between the internal network and the internet. The DMZ contains those systems which – due to a direct connection to the internet – cannot be fully trusted. These systems in the DMZ often do have some necessary connections to systems on the internal network, for example, in order to fetch data for webservers from back office systems.
The DMZ is an important part of the network to allow for a safe link to the outside world. Therefore, in setting up a new DMZ or making major adjustments to an existing one, it is wise to conduct a thorough review.