hacker

is a person who delights in having an intimate understanding of the internal workings of a system, computers and computer networks in particular, as defined by Request for Comments (RFC) 1392 [1]. More specifically, there are several different hacker subcultures:

• People engaged in circumvention of computer security. This primarily refers to unauthorized remote computer break-ins via a communication network such as the Internet (black hats), but also includes those who debug or fix security problems (white hats). Its earliest known meaning in the computer context^[1] referred to an unauthorized user of the telephone company network (now called a phreaker).
• A community of enthusiast computer programmers, originated in the 1960s around the the Massachusetts Institute of Technology (MIT)'s Tech Model Railroad Club (TMRC) and MIT Artificial Intelligence Laboratory. This community is notable for launching the free software movement. The World Wide Web and the Internet itself are also considered to be hacker artifacts. [2]
• The hobbyist home computing community of the late 1970s, focusing on both hardware (e.g. the Homebrew Computer Club) and software (the demoscene).

Nowadays, mainstream usage mostly refers to computer criminals, due to the mass media usage of the word since the 1980s. Unlike the definition in the RFC given above, this includes script kiddies, people breaking into computers using programs written by others, with very little knowledge about the way they work. Free software hackers consider this usage incorrect, and refer to security breakers as crackers.