A Brief "Malware" Primer
 (a term meaning evil software)
Some definitions courtesy of   Webopedia.com
 

• A computer virus attaches itself to a program or file so it can spread from one computer to another, leaving infections as it travels. Much like human viruses, computer viruses can range in severity; some viruses cause only mildly annoying effects while others can damage your hardware, software, or files. Almost all viruses are attached to an executable file, which means the virus may exist on your computer but it cannot infect your computer unless you run or open the malicious program. It is important to note that a virus cannot be spread without a human action, (such as running an infected program) to keep it going.  People continue the spread of a computer virus, mostly unknowingly, by sharing infecting files or sending emails with viruses as attachments in the e-mail

• Spyware is software that covertly gathers user information through the user's Internet connection without his or her knowledge, usually for advertising purposes. Spyware applications are typically a hidden component of freeware or shareware programs that can be downloaded from the Internet; (although the majority of shareware and freeware applications do not include spyware). Once installed, the spyware monitors user activity on the Internet and transmits that information in the background to someone else. Spyware can also gather information about email addresses and even passwords and credit card numbers

• Spam is generally unsolicited email advertising being sent to a mailing list, similar to junk postal mail.  In addition to wasting people's time with unwanted e-mail, spam also eats up a lot of network bandwidth..  A variety of techniques have attempted to curb Spam, but because the Internet is public, there is little that can be done to prevent spam from being sent, so for now, the only solution is to find ways to keep it from getting in to our inbox.

• Pop-up Ads are a type of window that appears on top of (in front of) the browser window of a Web site that a user has visited. In contrast to a pop-under ad, which appears behind (in back of) the browser window, a pop-up is more obtrusive as it covers other windows, particularly the window that the user is trying to read. Pop-ups ads are used extensively in advertising on the Web, though advertising is not the only application for pop-up windows

• A worm is similar to a virus by its design, and is considered to be a sub-class of a virus. Worms spread from computer to computer, but unlike a virus, it has the ability to travel without any help from a person. A worm takes advantage of file or information transport features on your system, which allows it to travel unaided. The biggest danger with a worm is its ability to replicate itself on your system, so rather than your computer sending out a single worm, it could send out hundreds or thousands of copies of itself, creating a huge devastating effect. One example would be for a worm to send a copy of itself to everyone listed in your e-mail address book. Then, the worm replicates and sends itself out to everyone listed in each of the receiver's address book, and the manifest continues on down the line. 
  
  Due to the copying nature of a worm and its ability to travel across networks the end result in most cases is that the worm consumes too much system memory (or network bandwidth), causing Web servers, network servers, and individual computers to stop responding. In more recent worm attacks such as the much talked about .Blaster Worm., the worm has been designed to tunnel into your system and allow malicious users to control your computer remotely

• A Trojan Horse at first seems to be free useful software, but will actually do damage once installed or run on your computer.  Those on the receiving end of a Trojan Horse are usually tricked into opening them because they appear to be receiving legitimate software or files from a legitimate source.  When a Trojan is activated on your computer, the results can vary. Some Trojans are designed to be more annoying than malicious (like changing your desktop, adding silly active desktop icons) or they can cause serious damage by deleting files and destroying information on your system. Trojans are also known to create a backdoor on your computer that gives malicious users access to your system, possibly allowing confidential or personal information to be compromised. Unlike viruses and worms, Trojans do not reproduce by infecting other files nor do they self-replicate.