It's strength lies in it's superior package management. With it, it is possibly to update a running system over the internet. Hence it is secure and uptodate without being forced to buy a new version. One can choose between three trees: stable, unstable and testing. 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 Distribution that build up on Debian, e.g.
Assets and drawbacks
+ Stable main tree
- Inappropriate for newbies
- Difficult installation
- Partly difficult configuration