When the industry was in its infancy, the term Penetration Testing was commonly used to describe a test whereby an authorised individual would attempt to break into a computer system. Supposably this would give a real life „what a hacker could do‟ review of the security of a system.
Nowadays Penetration  testing  is widely  used  to  describe  a  security  review  of  IT Systems. This might include reviews of components such as:

1、Network review
2、Application review
3、Source Code review
4、Host hardening review
5、Wireless network reviews
6、etc

http://www.insomniasec.com/publications/Increasing_The_Value_Of_Penetration_Testing-wp.pdf