What is the difference between a Transport Domain and a Transport group?

Hi,
one of my friend ask me this question and here i am sharing my views on this question:

Transport domain and transport group does not having relationship.transport group is concern with transport directory not with transport domain.

The SAP system containing the same transport directory are
called to be in same transport group.That means in your
three system landscape is having individual directory
concept then there are three transport group in your system
landscape. and if you are having concept of common transport
directory in your system landscape then there is one
transport group in your system landscape.

Whereas a Domain Controller is the one box or SID which
would control the transports going in from DEV to TEST and
to PROD. Also the TP command is run from the domain
controller.



Regards,
Gagan Deep Kaushal