I have heard that we can obtain the source code of Ubuntu and customize it as we wish. But where can I find the source code of Ubuntu, and what languages are used to write Ubuntu?
You really don't want to download the source code for all of Ubuntu. That's millions and millions of pages of text – literally. Instead, you'll get the source code for specific packages you're interested in, and that's very easy.
Ubuntu is written in lots and lots of different languages. C, C++, Vala and Python are highly popular, but all kinds of languages are used. Getting the source code for a specific package is very easy. As an example, let's download the source code for the Ubuntu One control panel. Open a terminal and run
apt-get source ubuntuone-control-panel
You now have the source code for Ubuntu One Control Panel in a directory called ubuntuone-control-panel-VERSION_NUMBER.
Ready to explore? :)
While Ubuntu is an operating system, it is actually comprised of several different aspects.
One aspect is the kernel, the extreme back-end of the operating system.
Another aspect is the desktop manager, window manager, etc.
Then of course there's the programs.
Generally speaking, the source for all of these aspects is all open, and freely available. Most of these have their own projects, and you can download the source code from those projects specifically rather than have to mess around with source packages and stuff.
The packages and their sources are all available through Launchpad.