A hacker is a skilled computer programmer who uses her abilities to break into the computer systems of companies or organizations.
While most people imagine a hacker as a bad guy who steals credit card information, destroys data, or is otherwise malicious and destructive, some hackers break into systems simply to test their security. There are so many hackers today that they've formed a true community and subculture. The earliest use of hacker is from 1983, from hack as "person hired to do routine work," although MIT students in the 1960's reportedly used the word hack to mean "creative prank."