Computer hackers write, use and modify software to break into computer systems, often exploiting flaws in another programmer's code.
The security troubles that have dogged Microsoft's Internet Explorer Web browser, for instance, are caused by hackers writing pieces of code that exploit vulnerabilities in IE's code. These enable the hackers to use the browser as a springboard to carry out a malicious action, such as hijacking a user's PC.
Although many hackers are malicious and intend to cause disruption or hijack PCs for their own ends, some can simply be out for the challenge of cracking a particular security system. In the corporate world, so-called ethical (or professional) hackers are even employed to probe corporate security systems for flaws--a business known as "penetration testing."
Notable hackers include Gary McKinnon, the so-called "NASA hacker" and Kevin Mitnick, who served five years in jail for his exploits. Since being released from jail, Mitnick has carved a career for himself in the security industry--a path followed by many former hackers.
Natasha Lomas reported for Silicon.com in London.