Worms can be divided into the two classes:
1. Worms, which are run by themselves without your knowledge.
2. Worms, which are activated only when the user opens an attached file.
The first type includes worms, which use errors in mail programs. Such errors are common in the most popular email programs like Outlook Express. For example, many viruses use error IFRAME, which was identified long ago by Microsoft and fixed by installing certain patches. However, worms utilizing this error are the most common.
Worms of the second type rely on the fact that user, for some reason, will open a file attached to the e-mail. In order to encourage the user to run an infected file, creators of worms use different techniques. The most common method is to replace some important documents with an infected file. Almost always worms have "double extensions". In this case, the attached file has a name like: Docl.doc.pif, pict.jpg.com. This principle considers that the e-mail clients do not display the full file name and user will not see the second real extension. When the user thinks that the file is a document or image, the file actually is an executable (with extension exe, com, pif, scr, bat, cmd). When the file is opened, the worm's body is activated.
Complete picture includes other varieties of viruses, such as macro viruses, Trojans and utilities for hidden administration. Although they do not belong to the worms, they can be distributed by the email.
Nowadays email users receive hundreds of spam email on a daily basis. Even though some email servers have inbuilt spam filter, some of the letters are still able to reach users’ mailboxes.
Spam letters usually are easy to determine. Typically, sender’s email address looks odd and suspicious - sd2fdsdf@dfdf.com, ddf243@hfyr.com, gdf2gd56@ggggg.com. But some spammers use "human" addresses. Sometimes it come to the point, where spammers use the address of a person from a famous company - Microsoft, IBM, etc. If you see that the sender email address has similar characteristics, it is better to delete such message immediately.
Subject is also an important criteria. What would you think, if you receive an email with the subject "Re: answer to your question?” And this message comes from the email address which you have never used and have never seen before? This is spam.
The vast majority of spam is used for advertising. Usually spammers advertise product or service, sometimes to increase the number of hits on the web site, more seldom for the viruses or Trojans distribution. But the common goal is to bring their information to the greatest possible number of recipients at a minimal cost. And the sender does not care about the composition of the audience, number is the most important.
Sources: http://en.wikipedia.org/wiki/Spam_%28electronic%29
http://www.online.tusc.k12.al.us/tutorials/viruses/viruses.htm
http://www.youtube.com/watch?v=FXpmgZAjP0o
http://www.cartoonstock.com/directory/c/computer_worms.asp
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