Notepad Plus Plus supporting various functionalities which you can't find in normal notepad and wordpad provided by windows. For example, Support for tabs is key, letting users manage different documents simultaneously, Line-numbering, Built in Spell Checker etc.
The interface is very simpler. A familiar row of icons with their mouse-over labels sits at the top, and one row of tabs is just below that. The rest of the screen is made of nothing but pure white space. [As a programmer I feel good to see codes in notepad with lines.]
If you are dealing with Notepad for day to day task, its nice replacement.
You can find more information regarding this on http://notepad-plus.sourceforge.net/uk/site.htm