Viruses and Trojan Horses
Now Trojan horses, these are a "Door way" from your computer and account from someone else's. these usually are very discreet and seems legit, like a bank asking for confirmation on your account, but to be sure if it's real or not try to call the bank about the email.
When it comes to avoiding trojan horses, usually all you have to do is not respond if you don't know who it is or what it is that they sent you. Besides these examples all you have to do is be wary of what you click on and use common sense.