What is a QR code and how to read it in Android/iOS/Windows Mobile/Symbian/Java devices

I think that everyone knows about QR code. Even if there are some people that still don't know about QR code, then I will explain.
Maybe you have seen in your city, in walls pinned papers with an unusual form with cubes in them.

Above is given an example of QR code. So exactly they contain URL links to a website which can be set from the creator of QR code. So with QR codes, you can share a website link with peoples. So how they can get the link because there are no any URL, only unusual cubes?
There are special apps that can read QR codes and redirect people to the website you have specified in that QR code.
In the example given at the top, is a link to Cyber Programmers website.
So now I will give the app names that you can download in your smartphones and read the QR codes. 

QR code reader for 

Android - NeoReader
iOS - QR Reader
Windows Mobile - QR Code Reader
Symbian - QR Scanner
Java - Kaywa reader