A Hacker is a person who seeks deep understanding of the inner workings of computer systems: operating systems, software, security, programming, networking and the Internet. This includes seeking knowledge for overall better usage computers be it for general security or practical reasons. Many Hackers have been responsible for some great computer systems and software.

Hacking is the activities associated with a Hacker. These activities include checking for flaws in the security of operating systems (making sure that private information can be protected, passwords of users are not easliy uncovered, and that the system is "stable"), preventing / understanding viruses (learning how to contain and technically even how to create), network intrusion detection (knowing how to prevent, and detect Crackers), etc.

A Cracker is very much like a Hacker; many posses the desire for deep knowledge of computer systems. However, Crackers are distguished by the fact that their use of their knoweledge is malicious purposes; bringing down a website, extortion of a company's private information, etc. Crakers are responsible for creation of viruses (technically many Hackers wanting to make a name for themselves will write viruses), defacing of websites (changing homepage links to porn sites, etc), wares (software cracks for commercial software without permission of the company), etc. They are generally not respected because many just use tools found on the Internet and posses any actual knoweledge.

Hacking is a legal profession / hobby. It becomes illegal and considered Cracking when the reasons behind the "hacks" are executed for malicious reasons or withour permission of the "victim". Technically / Generally, Computer Security Anaylist are often considered to be Hackers because they are legally hired to inspect a company's security infrostructure. Many Hackers (including myself) become upset when seeing / hearing about how some "hacker" stole some credit card numbers or broke into a website, etc. because these are not the actions of Hackers. They are the actions of Crackers or ankel bitters.