Hacking involves gaining unauthorized access to computer systems to learn about their internal workings or exploit security vulnerabilities. The document profiles three infamous hackers - Gary McKinnon, who hacked into NASA and Pentagon systems; Vladimir Levin, who stole $10 million from Citibank; and Kevin Mitnick, the "most famous hacker" who was the first to serve prison time for computer infiltration. All three faced legal consequences for their hacking activities.