This is how all happened. I was trying to search good screen recorder and found on some website that recordMyDesktop will work well under GNOME *Shell. So I tried searching in google "how to find whether I'm using gnome shell". The first link directed me to official gnome wesite there it showed a message as below.
So I thought it is something to do with desktop environment and continue browsing as below.
In this question I tried first answer to find my desktop environment, but the command DESKTOP_SESSION gave me "command not found". So I tried 5th answer by Nadiw. The command and output are as below.
ls /usr/bin/*session /usr/bin/ck-launch-session /usr/bin/gnome-session
So it is confirmed I'm using GNOME but it is not showing as desktop environment instead its showing "session", by which I got confused.
Hence I tried third answer by Luis Alvarado from this post. The command and output are as below
echo $XDG_CURRENT_DESKTOP Unity
Now I found this totally irrelevant. My questions are
If the desktop environment is Unity how can it use a session of GNOME and GNOME shell.
How GNOME shell is different from bourne shell, t shell and others
Or if one of the outputs I'm getting is wrong? Then how to find which DE i'm using? And which is the default DE for Ubuntu 14.04.
On which of these factors I should depend while downloading a software? Whether it is Desktop Environment or session or shell?