DMZ is basically a "zone" where you can specify one or more (depending on the router) machines to be more or less opened up to the greater network (not behind NAT).
Wikipedia defines it as:
a physical or logical subnetwork that contains and exposes an
organization's external-facing services to a larger and untrusted
network, usually the Internet.
This is where you would specify a machine that you want publicly accessible without having to configure port forwarding rules in order for traffic to reach it behind a router that has NAT enabled. This means that it is completely open and accessible on all listening ports - so if you do this, double-check what services you have listening and make sure you want them all up. For security reasons, this state also removes it from normal internal network status, so only limited connectivity is provided between this machine and those on the internal network (not all routers work quite like this, some SOHO routers will allow it normal access to the internal network, which is more of a security risk).
As to whether or not it will solve your problem (and not create more), would depend a bit on exactly what equipment you're using and exactly what you're trying to do. For the most part, you're probably good with just port forwarding a single service and not worrying about a DMZ.
ok that is clear about DMZ, but i have DDNS enabled in my router i cant access it via internet just i can access it in my network, and i make ip in dns to equal the ip in browser it mean public, but also cant access it not form lan or internet, but when ip in dns is mine router ip its ok for lan accessing but i cant access it from internet – chikoo711 – 2014-03-21T13:54:12.783