Whats the difference

Whats the difference of trojans, virus, spyware, malware, and worms? I know that not all programs can clean each one out. But why do some programs get some and some dont. You would think they make it to where it get it all. Its like you got to download a program that says trojan remover on it, then same for virus and spyware and malware, and scan the computer 4 times with different products to try to get them. Iam just asking for some clearity on what is what and how come programs dont all catch... and how do you possible get them from accepting file, clicking add, going to wrong site. What and I am just wondering because I wanna know what they are and why theres so many programs to download and scan for each.

 

The articles in these links will shed some light on what the differences are and therefore why different software will/may address them separately.

http://www.webopedia.com/DidYouKnow/Internet/2004/virus.asp

http://service1.symantec.com/support/nav.nsf/docid/1999041209131106