BackTrack is a distribution based on the Debian GNU/Linux distribution aimed at digital forensics and penetration testing use.
It is named after backtracking. The current version is BackTrack 5 R3 now based on Ubuntu 10.04 (Lucid) LTS, which is itself based on Debian.
Penetration Testing is a testing technique for discovering, understanding, and documenting all the security holes that can be found in a system.It is not a proof techniques. It can never prove the absence of security  flaws. It can only prove their presence.