Question
I am a web developer and usually use PHP, JavaScript or MySQL.
I have heard lot about Python. But I have no idea where it is used and why it is used.
Just like PHP, ASP, ColdFusion, .NET are used to build websites, and C, C++, Java are used to build software or desktop apps.
Where does Python fit in this?
What can Python do that these other languages cannot do?
Solution
Python started as a scripting language for Linux like Perl but less cryptic. Now it is used for both web and desktop applications and is available on Windows too. Desktop GUI APIs like GTK have their Python implementations and Python based web frameworks like Django are preferred by many over PHP et al. for web applications.
And by the way,
Solution
All the languages you've mentioned are Turing Complete, so in theory there is nothing one can do and another can't. In practice of course, there are differences, especially in productivity and efficiency. Compared to C, C++ and Java, which are static typed, Python is a dynamic language and can help you write the same code in significantly fewer lines. Python has a moto "batteries included", which means that the standard library offers all the things needed to build a complex application. Other languages would need external libraries for this. On top of this, since Python is an old and mature language (older than Java), many external libraries (for game development and scientific calculations just to mention a few) have been evolved. So Python can be used to program desktop applications and in fact in some cases more efficiently than other traditional languages.
Python is also a scripting language. This means that you can easily and quickly write scripts and simple tests with it.
More recently python is also used for web frameworks. Since there is a big code base and many python programmers, this was a logical thing to do. These web frameworks follow the practice mainly introduced by Ruby on Rails.