In the search engine world, spam is defined as the manipulation of a web page to give it an artificial boost in the search engine rankings. Generally, spam is what the search engines say it is. I.e., it is defined by the search engines themselves. Each of the major search engines provide specific guidelines describing what webmasters should and should not do to their web pages in order to achieve a better search engine ranking, though that has not always been the case. Several methods are universally deemed search spam, including:

1. Hidden text/Hidden links. Hidden text is textual content which your visitors cannot see, but which is still readable by the search engines. The idea is to load a Web page with keywords and keyword phrases that would be unsightly to visitors but that would improve the page's rankings in the search engine results, and to do so without letting your visitors see the text.
2. Keyword repetition
3. Doorway pages. Doorway pages are pages that are created for the sole purpose of pointing traffic toward your site. They are usually only one page in length and the design tends to be quite simple.
4. Mirror pages. A mirror site is simply a copy of your site. It's usually your main page. A mirror site looks exactly like the original.
5. Cloaking

We recommend not using the above listed unethical search engine optimization techniques. For engine-specific guidelines, visit the search engines themselves.

Google Information for Webmasters - Quality Guidelines

Suffice to know that search spam might sometimes work for a while, but sooner or later the search engines will work out such unethical, get-rich-quick techniques and then blacklist you from their results.

Once blacklisted (or sandboxed) it is very difficult to rebuild your reputation with the search engines again. So, avoid these manipulation techniques completely or be prepared to have your online business collapse.