This is any kind of software that could hurt your computer
Viruses can get sent to you in different ways but if you dont open it and it doesn't attach itself to anything on your computer it can not spread.
These are very difficult to deal with because they can be added on to something that you wanted. For example, if your downloading a song you could get your song and a virus.
These are types of software that looks real but secretly obtains personal information from you.
Ways to Avoid This
Anti-virus software is a software that you can buy to protect your computer. Also, you could buy a firewall and make sure to use common sense