Malware
Viruses, Trojan Horses, and Spyware
Viruses
Viruses are a kind of Malware that automatically copy themselves, however needs contact (such a clicking a link) to activate. They can be found in emails from unknown people or by clicking on a link stating that you won something.
Trojan Horses
Gets into your computer by downloading something that may seem legit, but allows access to your computer. You can get them by downloading anything - a picture, music, a video, etc.
Spyware
Spyware tracks everything that a user does or types on a computer. It can be on a thumb drive or disk at a public computer, and everything will be tracked. However, it can also be put on your computer through download.
How to avoid it
You can avoid getting these on your computer by installing antivirus software that warns you if there a potential virus, putting up a firewall that has to accept all access that your computer has with the internet, and by using common sense!