They are all connected to some degree. For the average user, you do not need any of them and most cases all of them. However if you need to connect a security DVR, cameras, game machines, etc then they come into play. Best to use the minimum required to get your device to work. So if you have a specific device to connect, then work on getting that connected.

I am not more technical guy but a little bit technical that's why I am interested to know these stuffs so that I can learn and know how to use them. Pls let me know some scenarios for all individually so that I can understand where these are (Dynamic DNS, Virtual Server, Port Triggering and DMZ) used?

I never needed DMZ or port Triggering so do not know much about them. I use address reservation to reserve one of my IP address for a camera. Then use the Virtual Server to link that IP address to a set of ports. Somethings such as a security DVR require more than one port. I use a DDNS service to get to my Security cameras from outside of my network. I set up my IPv4 address (whatismyip.com) on the DDNS web site (such as ddns.com) with a unique name for example xxxxxx. So on each of my iPads in the app for the camera, I use the register mode of IP/Domain and an address of xxxxxx.ddns.com and the device port number in the app for the cameras. So when I connect from outside with my iPad, it can get my IPv4 address go to that port and see my camera. If the IPv4 address changes, only the web site needs changing instead of all of the apps in all of the devices. There are functions in many routers to keep that address up to date although they may restrict which DDNS provider you use. There are also programs for your computer to keep the address updated for some of the DDNS providers.