Hi Dhirish,
Transport groups is basically where all R/3 systems that access a common transport directory.
It's something like that you want to move a transport request from DEV to 5 clients in QAS where your transport groups help.
With reference to structure and details you can find more details with below url.
http://help.sap.com/saphelp_46c/helpdata/en/c4/6045377b52253de10000009b38f889/frameset.htm
Regards,
Deepanshu Sharma