I like to know what is hacking is in computer?
Please help me...

If you are looking for a definition, then it's basically what you see below that is from the following site. define:Hacking - Google Search

Definitions of Hacking on the Web:

• Unauthorized use, or attempts to circumvent or bypass the security mechanisms of an information system or network.
  www.tsl.state.tx.us/ld/pubs/compsecurity/glossary.html
• Accessing illegally or without authorization computers, accounts, equipment or networks belonging to another party, or attempting to penetrate security measures of another system.
  www.searcycafe.com/wifiaup.php
• Hacking means illegally accessing other people's computer systems for destroying, disrupting or carrying out illegal activities on the network or computer systems.
  www.infosec.gov.hk/english/general/glossary_gj.htm
• The US Dept. of Justice defines Hacking as “All illegal access to a computer or a network”.
  www.root777.com/computer-security/important-computer-security-terms-and-terminology/
• Hacking originally referred to programming. A quick and/or clever creation of a program or solving of a problem was referred to as “a hack”. A particularly adept programmer might be said to be a “good hacker”.
  www.earthlinksecurity.com/security/articles/glossary/