Ext JS is a commercial product. It means you have to buy it in order to develop any product which is intended for sale. However, if you want to use Ext JS for learning purpose or to build open source product then you can download GPL sdk.
Download latest version of Ext JS SDK (which is Ext JS 6 as of this writing) from https://www.sencha.com/legal/gpl/. Provide your mail id and acknowledge terms and condition to download it free.
Ext JS 6 SDK includes following folders.
Ext JS 5 SDK includes following folders.
Ext JS is used to build web based enterprise applications. So, you need to install a web server on your local machine. You may use any web server of your choice. However, some IDEs have built-in local web server so you can use that too.
If you are using windows platform then you can use IIS by turning on IIS features. Visit Technet to download and install IIS 7.5 on windows 7.
Once you have installed Apache or IIS, you can verify that it is running by navigating to localhost in your browser. You should see a startup page indicating successful installation and running of Apache HTTP Server or IIS welcome page.
IDE should satisfy following requirements for the development:
Learn how to setup Aptana, Eclipse and Visual Studio in the next chapter.