Final answer:
The term for someone skilled at accessing computer systems, sometimes unlawfully, is 'hacker'. They use various tactics like phishing and social engineering to steal data, which they may exploit for gain. Countermeasures include strong security practices and user education.
Step-by-step explanation:
A person who is skilled at gaining access to a computer system and sometimes does so illegally is called a hacker. Hackers often use various tactics to infiltrate systems, including phishing attacks to get individuals to click on malicious links, exploiting software vulnerabilities, or using social engineering tactics to manipulate people into divulging sensitive information. Once they gain access, hackers may steal personal information, intellectual property, or even disrupt services. They might use the stolen data for identity theft, financial gain, or to sell on the dark web.
Hacking can be quite successful if not properly defended against. To stop or reduce hacking, organizations and individuals should implement strong security measures such as regular software updates, using complex passwords, employing firewalls and antivirus software, and educating users about security best practices. Technologies and strategies always evolve in this ongoing battle between hackers and defenders.