#defluff

This is a defluffed post. Majority of the content comes from an original source, summarized for brevity.

Original Source

Most of the terms we are used to calling virus or spyware is technically not that. It's a – 

Malware – a generic term used to describe all of the hostile and intrusive program codes including viruses, spyware, worms, Trojans, or anything that is designed to perform malicious operations on a computer.

All the other terms are just methods of malware delivery or infestations.

Virus – a program that self-replicates after hooking itself onto something running in Windows®.

Worm – another kind of self-replicating program but generally doesn’t hook itself onto a Windows process. Worms generally are little programs that run in the background of your system.

Trojan – software that you thought was going to be one thing, but turns out to be something bad. Named for the fabled “Trojan Horse” that appeared to be a gift but in fact carried a dangerous payload.

Drive-by download – this is probably the most popular way to get something nasty into your computer. Most of the time, it comes from visiting a bad web page. That web page exploits a weakness in your browser and causes your system to become infected.

Malware could be any of these –

Adware – software that uses some form of advertising delivery system. Sometimes the way that advertisements are delivered can be deceptive in that they track or reveal more information about you than you would like.

Spyware – software that monitors your computer and reveals collected information to an interested party - tracks what webpages you visit or monitors everything you do with your mouse and keyboard. 

Ransomware – lately a very popular way for Internet criminals to make money. This malware alters your system in such a way that you’re unable to get into it normally. It will then display some kind of screen that demands some form of payment to have the computer unlocked.