A computer virus is a type of malicious software designed to replicate itself and spread from one computer to another. It attaches itself to legitimate programs or files, and when these are executed, the virus activates and can cause harm to the system, such as deleting files or stealing information. Viruses can spread through various means, including email attachments, downloads, and infected USB drives. To protect against them, users can install antivirus software, keep their systems updated, and avoid opening suspicious links or files. Regular backups of important data are also recommended to mitigate potential damage.