Computer security involves protecting computing systems and data from unauthorized access or modification. It aims to ensure integrity, confidentiality, reliability, and authentication of data. Complete security is impossible as networks carry both legitimate and malicious traffic, making computer security an ongoing challenge similar to crime prevention. Common security threats include password guessing, phishing, spoofing, backdoors, buffer overflows, and denial of service attacks. Protecting computers requires techniques like encryption, secure networks, antivirus software, firewalls, and safe computing practices for users.