Hacking involves gaining unauthorized access to computer systems or networks. There are three main types of hackers: white hats who hack for security testing, black hats who hack maliciously, and grey hats who may hack to expose vulnerabilities but also for profit. Common hacking targets include passwords, emails, websites, and credit card numbers. While hacking can be used to recover lost passwords or test security, it is illegal and poses disadvantages like enabling criminals to steal data, violating privacy, and allowing competitors to destroy work.