Viruses, worms, spyware... what's it all about? If you're a novice computer user, the variety of terms surrounding "bad" computer software (known collectively as malware) might confuse you to the point where you're too paralyzed to do anything to protect yourself. That's a mistake. So let's start by covering the bases and laying out what's what in the world of evil apps.

Virus - Technically a specific type of program that replicates itself from one computer to another by infecting an application, often doing damage along the way (but sometimes can be harmless). True viruses aren't actually that common any more (making up only 5% of malware infections), but the word "virus" has largely become a generic term to describe any form of malware.

Worm - Like a virus, but a worm doesn't need an application to infect to replicate itself. Much more common than a true virus these days.

Trojan - Trojans, or Trojan horses, make up the majority of malware infections today. Trojans are designed to look harmless (or look like nothing at all, installing themselves in the background) but actually go to work behind the scenes installing additional malware or performing nefarious activities like sending spam or grabbing your personal info. In contrast, worms and viruses are generally noticeable immediately.

Back Door - Also written as backdoor, this malware type is designed to give remote control of your PC to another user, over the internet.

All four of the above terms are used to describe methods by which malware gets on your PC. Once the malware gets on your computer, it can take any number of forms, as outlined below. 

Bot - Any of the above malware types can turn your computer into a "bot," also known as a zombie, one of the biggest security problems on the web today. Bots automatically perform nefarious actions like sending spam or firing off denial of service attacks (which aim to bring down web sites through a crush of traffic), all under the command of a botnet's master computer. As a user, you may never even notice a bot is installed on your PC, but in quantity, bots can be very dangerous to other computers on the web.

Spyware - A broad term for applications that steal info off your computer (like your address book or your passwords) and transmit them to the bad guys. Some spyware is relatively harmless, but at its worst, spyware can be extremely damaging and can result in identity theft.

Keylogger - A type of spyware that captures characters as you type them as a means of recording your private correspondence. Keylogger hardware also exists.

Downloader - Downloaders: Well, they download stuff. Typically, they download more malware, which in turn download even more malware. This is how a single malware infection can spiral out of control and turn into dozens of infections on a single PC to the point where the computer becomes wholly unusable. Many malware infections come with a downloader component.

Dialer - Got a modem? A dialer will run up a fat bill for you by auto-dialing 900 numbers (and similar toll numbers) while you sleep.

I'm leaving out some less common malware types, but those are the basics. It may sound overwhelming, but the good news is that no matter what type of malware infection you're facing, the prevention method is the same. And that's a subject we'll cover later this month... Stay tuned for Part 2: Prevention.