The term hack may have several definitions. It may refer to a exploit used against a computer system.
One commonly used meaning of the term hack is the act of breaking into a computer. Usually a hack is performed by utilizing a software vulnerability that allows a third party to get control of a user's computer without their consent. A hack may also employ a stolen password. Stolen passwords may be obtained by snooping on a network or watching people type their password. A keylogger program may be used by an attacker to obtain a password without the user's consent. Some passwords may be obtained in their encrypted form and then decrypted or broken by the attacker.
Although the act of hacking is usually thought of as an act performed manually by one individual, many times it is automated by programs today. Viruses may try to exploit known vulnerabilities and this is a computer hack. Also other progams such as adware or spyware may get installed on a victim's computer using a computer hack.