Viruses, Trojan Horses, and Spyware
Any malicious software. There are three types of Malware, Viruses, Trojan Horses and Spyware.
They copy themselves and spread. The need contact to spread though. Contact as in through email, web sight links, etc.
Just like in the stories, it seems friendly but just opens a door for people to access your personal info.
Spyware secretly takes all your personal information without you knowing.
How to avoid:
You can get an anti virus systems, a firewall, or just use your common sense.