Viruses and some types.

Viruses:

It is a program or a piece of code loaded onto your computer without your knowledge and run against your consent.
A virus is attached to any file or program itself and spreads from one computer to another, leaving infection as travel.
All viruses are attached with a (.exe) type file, which means the virus may be on your computer or device but, that can run only when you or anyone run that.

A worm is similar to the virus and it is also a sub-class of the virus.
A computer worm is a virus that spreads by creating duplication into the Drive.
Computer worms can send copies of worms to another network, computer, or drive easily so that is more active than the virus.
The worm can send its copies to another computer through the network, Bluetooth, mail, or infected web pages.

There will be showing the new unknown icon on the desktop or any else other path.
Your computer becomes slower than before.

An intruder is someone that enters the place without any kind of permission or invitation. Also, it knows as hackers or crackers.

They pretend to be someone that he was not.
An individual who is not authorized to use the computer and who penetrates a system's access controls to exploit a legal user's account.
B. Misfeasor:
The authorized person doing unauthorized actions.
that was a permitted user.
A legal user did not have permission to access data or have access but not for that work, he/she misused his/her privileges.
C. Clandestine User:
A clandestine user is an individual who seizes supervisory control to avoid the system's security mechanisms such as manager or access controls.

Example of Intruders: