difference between gnu system and gnu software

Developing a whole system is a very large project. 
To bring it into reach, I decided to adapt and use existing pieces of free software wherever that was possible.

For example, I decided at the very beginning to use TeX as the principal text formatter; 
a few years later, I decided to use the X Window System rather than writing another window system for GNU.

Because of this decision, the GNU system is 
	not the same as the collection of all GNU software. 

The GNU system includes programs 
	that are not GNU software, 
	programs that were developed by other people and projects for their own purposes,
but that we can use because they are free software.