A marketing tactic that involves preying on the naive masses. Typical examples of viral marketing include:

Almost all viral marketing is outsourced to specialist companies. Let's imagine our product is Aloe, some random new piece of hardware created by Vera Inc.

Newsgroup posting
Accounts are created expressly for the purpose of posting pro-Aloe messages. This isn't like traditional spam, as it's not repetitive or obvious. Some attempts are made to make the posts appear 'genuine'. The level to which this is taken can vary. Some services keep long standing genuine accounts which are used by real people most of the time, and only occassionally used for pro-Aloe messages. Thus making the recommendations appear much more genuine.

Similar to the newsgroups, IRC identities are created specifically to generate conversation about Aloe. This can be very effective in kick-starting word of mouth promotion. Again, like the newsgroups, identities vary in the strength of their cover.

Gimmick Promotions These centre around some kind of novelty, such as an email attachment game, interactive section to the website, 'special offer' requiring you to 'recommend' the email addresses of friends in order to get a discount. In terms of generating traffic these can be the most productive tactics. Many variations on this exist, and chances are you've received or seen some form of it at some time, athough you may not be aware of it.

Search Engines
There are a number of services available that can boost Aloe and Vera Inc's ranking in search engine results. Many tips to achieve this are available on the web. But Vera Inc is more likely to outsource this to a company that has a proven technique or product for manipulating search algorithms. This is less of a viral tactic than the others.

Viral marketing is just another marketing tactic, in my opinion a cheap and annoying one, but you can't deny it's effectiveness. Conspiracy theorists can get carried away with the witchhunt - "Am I the only real person on USENET? What if everyone else is just a viral marketer?" etc. It has been known to seriously backfire, causing the parent company much aggravation as they are powerless to stop the flood of abusive USENET postings.

And then of course there's the stigma of being laughed off IRC ..