Viruses, Trojan horses, spyware
What are those?
A viruses are things that copies it self but needs to spread by contact like through email. A trojan horse is something that looks really good but once its in it turns into something else. Spyware it secretly collect personal info on you.
How to stop them?
There are a few ways to stop these you can have a firewall, antivirus software, or the most important is common sense. If you see something that wants personal info call the company that they are saying they are and ask if they really need the info. If soemthing looks to good than it is, don't open it.
Why is it bad?
All of these things are bad cause they can destroy your computer and or get personal information on you.