To expand a bit more on Amit's reply, NetBeans is an IDE. An IDE allows you to write code - easily. You can just as well do it in notepad, but an IDE would give you features like code completion, syntax highlighting, in-program compilation, debugging, etc. Ofcourse, there are many more features that NetBeans has, you can check them out of the NetBeans website.
The advantage is that it makes your coding easier. Also, programs are conveniently grouped into "projects", and not as individual source files, hence maintaining a logical flow. NetBeans also includes a WYSIWYG editor for making Swing GUI apps. There are many plugins which can be used to extend the features of NetBeans.
Nope. Netbeans is mainly a java IDE, meaning that you can use it to develop any java code (console, mobile, web, EJB, etc). It also supports developing ruby and C/C++ [ April 22, 2008: Message edited by: Alaa Nassef ]