First off, how do you get viruses in the first place? Ever opened an email of someone you didn't know? Ever been redirected to a website you wasn't to sure was safe? Ever got a popup telling you that you have a virus and "click here" to fix the issue? These are common tactics people fall for in order to get a virus and usually end up paying a lot of money in order to get rid of them. Viruses can really harm your computer if not left in check. Most times you aren't even aware you have a virus until something eventually catches your attention. I will explain in further detail what types of viruses there are.
Trojan Horse: A virus that pretends to be a legit program that waits for you to run it. It then install itself onto your computer and causes havoc. What makes a Trojan horse unique is that it doesn't replicate.
There are many many types of Trojans that preform different tasks. Some are key-loggers, ransom-ware, backdoor, and banker Trojans. The list goes on and on and they are constantly being developed to make our lives a living hell.