Viruses, Trojan Horses, Spyware
A (computer) virus is like a disease that grows and duplicates itself over and over again. Once a virus starts, it is very difficult to get rid of and can take a long time.
A "Trojan Horse" is something on the Internet, like a picture, video, or document, that look real but isn't. If opened, a Trojan Horse could give your computer a virus.
Spyware is certain equipment that is used by hackers to receive or collect information from public or even private computers.