Back when search engines were more primitive, it was easy to fool them into increasing your rank order by using "not quite honest" methods. I'm sure you've heard of some of these tactics, however they are not effective anymore. Employing them can actually lower your ranking and get you banned from the search engine all together. Such illegal tactics, similar to its email counterpart, is called spamming. Here are some of the tactics to be cautious of

Doorway Pages
These are separate web pages usually filled with keywords, content, or anything that the promoter thinks will improve rank order. These pages may or may not be linked to your web site. They are only intended for search engines to see. The idea is that the more web pages there are within the site, the more likely it will be that such pages will achieve higher rank order. Search engines today can detect this.

Cloaking
Suppose that your SEO offers 100 or so domain names that will be used to promote your web site. Each site is filled with content similar to doorway pages and is submitted to the search engines for indexing. When someone does a search and stumbles upon one of these cloaked domains, they are redirected to the site they were intended to see. The problem with this is that only SEOs usually have the time and resources to implement cloaking tactics. So if the relationship sours with the SEO, they can terminate all cloaked sites or better yet redirect the domains to a competitor's web site. (This actually happened with one devious SEO. (See Dot Com Noir)

Irrelevant Content
In the past you could fill your site with content that had absolutely nothing to do with yours in the hope that someone would find you by coincidence. For example, if your site sold a certain software application, you may try to generate traffic by using keywords such as computers, Microsoft, pda, keyboards, mouse, etc. Obviously the site has nothing to do with Microsoft or hardware but the idea is to lure someone looking for computers to this site and grasp their interest. Sounds like a good idea, no? Not only is it dishonest but also search engines have wised up to this tactic.

Hidden or Redundant Content
Webmasters used to fill their pages with repeated keywords and mask them by making the text the same color as the background. Search engines can detect this now and will discount this content. Some sites allow you to see the keywords. You probably have come upon a page that had a lot of junk at the bottom. Search engines can detect such content with consistency.

Abuse of Meta Content
Meta tags are coded into the HTML of each of your web pages. The Metas are a convenient way of including keywords and description about that particular web page into your code to assist the search engine in indexing content. Many times, webmasters will repeat the same keywords many times in order to increase relevancy. One time I viewed the HTML source of a web page and I was shocked to see three pages of keywords. Despite the fact that this worked fairly well in 1997, it would be immediately discounted today.

Link Farms
Links farms are web site communities whose sole purpose is to link to each other's site in the hope of increasing popularity. You scratch my back, I scratch yours, right? The problem with this is that these link farms have no purpose other than to exploit the search engines. The links have no relevance to content thus degrading the usefulness of the search engine. The technique works only until the farm is discovered whereby every site within the farm can be instantly banned.