Hacking is the process of writing code for, or contributing code to, a piece of software.
There is some controversy surrounding the meaning of this term. It began as a benign term meaning "to exercise proficiency" or "to alter or improve," but the popular media have since construed it to mean "to break into a computer system, usually with malicious intent." Many in the computer industry have recently begun trying to 'take back' the word from its popular mutation, and many have adopted the term cracking to replace the malicious interpretation. Because of the desire to reclaim the word, you will often find the term used in conjunction with open source projects, intended in its benign form. For more information about the history of the term, please see Wikipedia's article on Hacker.