Its strength lies in its superior package management. With it, it is possible to update a running system over the internet. Hence it is secure and up-to-date without being forced to buy a new version. One can choose between three trees: stable, testing and unstable. So it's up to oneself how stable or experimental ones own system is. Also Debian is available for 12 different hardware platforms.
There are many distributions that build upon Debian, e.g.
Assets and drawbacks
+ Stable main tree
- Inappropriate for newbies
- Difficult installation
- Partly difficult configuration