Hacker is a term used by some to mean "a clever programmer" and by others, especially those in popular media, to mean "someone who tries to break into computer systems."
1) Eric Raymond, compiler of The New Hacker's Dictionary, defines a hacker as a clever programmer. A "good hack" is a clever solution to a programming problem and "hacking" is the act of doing it. Raymond lists five possible characteristics that qualify one as a hacker, which we paraphrase here:
A person who enjoys learning details of a programming language or system
A person who enjoys actually doing the programming rather than just theorizing about it
A person capable of appreciating someone else's hacking
A person who picks up programming quickly
A person who is an expert at a particular programming language or system, as in "UNIX hacker"
Raymond deprecates the use of this term for someone who attempts to crack someone else's system or otherwise uses programming or expert knowledge to act maliciously. He prefers the term cracker for this meaning.
2) The term hacker is used in popular media to describe someone who attempts to break into computer systems. Typically, this kind of hacker would be a proficient programmer or engineer with sufficient technical knowledge to understand the weak points in a security system. For more on this usage, see cracker.
In other word
"1.a person who uses computers to gain unauthorized access to data.
2.a person or thing that hacks or cuts roughly."