A friend reading this blog said that it was too techy for them, and since I intend to write this for the general public, I am going to publish some primer articles. Here is a completely off the top of my head explanation of popular security terms in no particular order. I promise to be brief.
a virus is a program written to make copies of itself (we say replicate) and sometimes to attach itself to other programs. Viruses were mostly written just to prove that someone was talented enough to write one. The vast majority of viruses serve no other purpose, and because of that, you never heard of them. Viruses are no longer the majority of attacks on your system but there are still plenty of them around. Less than ten percent of viruses ever did anything you could see. The term was coined by a grad student adviser at Lehigh University when Dr Fred Cohen was writing his dissertation. Viruses existed before that under a number of other names. If a virus has other purposes (data destruction, password stealing, downloading other malware) it is called a PAYLOAD. Some viruses have Trojan-like behavior.
A Trojan ( or Trojan horse program ) is a program that pretends to be something that it isn’t, in order to fool you into running it. It can also be a program that installs a logical ‘back door’ into a system. Some people say that a Trojan is a virus that doesn’t replicate.
Malware is all unwanted, destructive or malicious software. This term was coined by Yisrael Radai in 1990. All Viruses, Trojans, Worms, Bots and other malicious software are malware. Malware is the big term that includes them all.
A worm is a program that spreads itself from one computer to another, using a common medium (such as a network connection, email or web page). Since the worm must replicate itself in order to do that, worms are an entirely bound subset of viruses. The first use I know of the term is in a 1975 Science Fiction novel called Shockwave Rider by John Brunner.
NOTE: Not all virus researchers agree on this taxonomy, but after doing a lot of research, it is the most common. It is important when describing something, that we have precise and agreed upon words. I am always seeking to improve, simplify and come to agreement on this Nomenclature. It is my pet peeve.
David Perry–Huntington Beach–November 13 2014