I have searched a lot about it but could not get an answer. I read somewhere that linux Ubuntu has 6 or 7 terminals.
We can start them by pressing -
Ctrl+Alt+(F1-F6)
and to get back to GUI-
Ctrl+Alt+F7
So what i want to know is what is the difference between these 6 different terminals , and how can we use them as per to different purposes??
In fact there are not only 6 or 7 ttys, there are many more. You could see them, try
ls /dev/tty* | wc -l
All the consoles are not always active. You need to activate ttyN in order to switch to it with Ctrl+Alt+F N. You can activate any tty with openvt command also. See man openvt.
Reason why there so many ttys, may have historical baskground. In old days Linux was used in terminals (without X server).
To do multiple tasks at the same time it was indispensable to have more than one ttys.
due to any accidentally break, it was required to have another tty from where system could be monitored.
Even to read more than one man pages, different ttys were needed.
Nowadays for most users don't need that many — in fact most users never see anything but first 6 ttys to be available for CUI login and the virtual console that X is running on. As Rinzwind mentioned all these terminals are mostly identical.
But there are a few people who patch their kernel to allow more than 63 consoles(special ttys), because they run large machines with many hardware consoles.
There is no difference between the 6 terminals -by default-. It is up to the user to think of use cases.
On a server setup I have seen these used to automate a login after booting up to start software. It is also possible to have the desktop to show more than 1 desktop manager: Unity on tty7, KDE on tty6 and XFCE on tty5 (but installation wise it might be a bit more difficult than it used to be).
Virtual Terminals are provided as a convenience. They come in very handy when troubleshooting problems with the GUI.
On servers that do not have a GUI they are indispensable.
