There are many ways people try to attract traffic from search engines. Some aggressive methods that go too far are collectively known by a term I coined back in early 1996, "search engine spamming." Spamdexing and spammage are other terms applied to these methods. This page describes some techniques that have often been viewed as spamming. Also see the Search Engine Spamming Articles page for a compilation of other articles from Search Engine Watch and across the web on this topic.
Don't Be A Spammer
You shouldn't spam the search engines. The time you spend trying to trick your way into the results is probably better spent on developing good content for your web site, which is ultimately what the search engines have been moving toward rewarding. Beyond this, any keyword phrase attracting heavy competition -- and thus spamming -- will require a lot of work to ensure a page based on spam stays in the top results. This is time better spend on other promotional methods.
Spamming tactics are also no guarantee that you'll rise to the top. Spamming is a brute force method that doesn't necessarily work. I've seen many examples where pages with good content and smart, acceptable search engine design, do better than spam pages.
Another reason to avoid spam is that search engines may penalize your pages or exclude them entirely, if they detect spamming. Search engines operate a variety of systems to detect spam, which are sometimes referred to as spam filters. The also receive reports of spam from their users. So, even if you slip past a search engine's spam filter, your competitor still might drop a message to a search engine, telling them on an unsavory behavior they've discovered.
Finally, I don't know anyone who likes to get spam mail. Many companies that have used spam mail services also find they get a lot of hate mail and lose goodwill on the Internet by doing so. For this reason, I don't feel there should be widespread acceptance of search engine spamming. It can cause results to become less meaningful. In time, I think people will not like companies that practice it.
What Are The Rules?
Unfortunately, there's no official "rule book" to tell you what all search engines consider to be spamming. Each can define their own rules, and some of the rules they have aren't even published. That sounds frightening, but few people ever "accidentally" spam a search engine. Instead, it's those who are extremely aggressive with search engines that run the most risk of being labeled a spamming.
In lieu of an official rule book, this page provides a summary of things that are generally frowned upon by search engines. Consider it a check list of things to avoid.
This is the repeated use of a word in an attempt to increase a page's relevancy. For example, one might place this at the bottom of a page:
money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money money
Search engines are very wise to keyword stuffing. Some of them even analyze pages to determine if the frequency of a word seems out of proportion to normal, "relevant" documents. This helps them combat more sophisticated forms of stuffing.
Since keyword stuffing is easy to spot, spammers adopted a technique of setting the stuffed keywords to be the same color as the page background. In this way, the words as shown above would not appear. Some search engines detect for this now.
Another method of hiding stuffing was to use tiny text, or placing words in a small font size. Because of this, some search engines may reject pages that make heavy use of small font sizes. To be safe, you should avoid having pages that are predominately in a font size other than the default size. So a page all in font size 1 might be bad. I've seen examples where pages like that were ignored, then accepted when reset to the default size. This doesn't mean using small text is absolutely bad. It's just best to avoid having the page be almost entirely in a small font size.
Page Spoofing / Meta Refresh / Redirection
The meta refresh tag allows those visiting a particular web page to be automatically taken to a new web page. Spammers will optimize a page for a particular keyword phrase. This gets submitted to the search engines. Then, anyone clicking through to this page from its listing in a search engine are automatically taken to a different page. Often, the "final" page people reach has little content relating to the keyword phrase. To combat this, some search engines won't accept pages with a fast meta refresh rate.
There are no published times as to how fast is too fast, when using a meta refresh command. I would say that at least 10 seconds, if not longer, would be best. However, using a meta refresh tag set to any time period might get your pages rejected by some search engines.
Meta Tag Stuffing
Meta tags are a useful, "legal" way of adding keywords to a page without resorting to keyword stuffing. However, sometimes people place words in them that are completely unrelated to a web page. For example, one adult site used the words IRS in its meta tags during tax season, in order to catch some people looking for IRS information.
Page Stuffing / Duplicate Pages
Often, the same web page will be duplicated or slightly changed, then these variations all submitted to a search engine. If the page is successful for a particular keyword phrase, then all of its variations can dominate a search engines top listings. Some search engines eliminate duplicate pages. Others watch for heavy submissions, which is a sign of page stuffing.
Multiple Title Tags
In early 1997, a number of spammers were enjoying success by using multiple title tags in their pages. They would repeat the title tag several times. It wouldn't cause the page to look different in browsers, but some search engines were susceptible to it. Search engines caught on to this quickly, and it no longer has the success it once had.
Code Swapping / Cloaking
Sometimes a web page is optimized for a search engine, and then the code is immediately changed after a position has been attained. This is done to prevent others from imitating the success of the page. The original page that was submitted may be very different from the page that is swapped in its place.
A good search engine catches code swappers by revisiting on a regular basis. There's no way to predict when a search engine will return to a site, so a code swapper risks losing a position, since they can't tell when to put the optimized page back up.
Spam vigilantes and competing sites also catch code swappers. They take advantage of search engines with instant add URL features to resubmit pages.
Some advanced webmasters use page cloaking systems to alleviate problems with code swapping. Some search engines may consider the use of cloaking to be spam, depending on the type of content being fed to them. See the More About Page Cloaking page for more information.
Domain Spamming / Mirror Sites
While there are some legitimate reasons to have "mirror" sites, which are identical sites found under different domain names, operating such sites simply to increase your search engine traffic is generally considered spamming. If the search engines detect you doing this, they may remove all of your sites.
Getting "Blacklisted" or "Banned"
If all your pages have disappeared from a search engine, you may have been "banned" or "blacklisted" for spamming. The first thing to do is resubmit. If all is well, your pages generally should appear within a month or so. If you still don't show up, there might be some technical problems with your server. Perhaps you have a robots.txt file up that's denying search engines access. A change to using frames or pages light in HTML copy (in other words, pages that are all images) could also be a problem.
If none of these are the culprit, you can and should also get in contact with the search engines. Just tell them that your pages aren't showing up -- don't say, "Am I blacklisted?" If you haven't done anything wrong, they'll hopefully look into the problem and help you correct it. If you have been blacklisted, they may tell you so and perhaps give you another chance, depending on how serious the abuse was considered.