Malware
malicious software
Examples
Viruses: A damaging file that spreads itself.
Trojan horses: You download something, but it gives the person you downloaded it from access to your computer and information.
Spyware: Gets personal information.
How to avoid
Download antivirus software. It'll stop viruses in their tracks.
Get a firewall. It blocks malware.
Use your common sense. Don't download suspicious things, and be wary of foreign sites and emails.