Hacking is the violation of server or network security via exploitation of weaknesses in that security.
Some of the most common ways security may be compromised include:
- Social engineering, in which a person is persuaded to give up sensitive information such as passwords
- Exploiting unpatched or not generally known (0-day) security holes in software
- Brute force, that is, trying common usernames and passwords repeatedly until something works
Less commonly, hacking refers to the uncommonly creative solution of a software or hardware problem. Some see this original definition as the correct one and wish to preserve it, though they seem to be fighting a losing battle.
Detailed security questions and answers which do not fit at Server Fault may be welcome at IT Security Stack Exchange.