SEO Doorway … Door (meaning anything providing a means of access (or escape);) way (meaning way) if we add the two words that have Doorway (meaning entry or portal) Or the entrance (the space in a wall) through which you enter or leave a room or building; the space that a door can close; “he stuck his head in the doorway”

Using this technique, considered Black Hat SEO many people try to manipulate the search engines by creating ports of entry or Doorways, each optimized for one keyword, but without content. The goal of Doorways is to position itself in the search engines and when the user accesses one of them is directed to another page. Another form of Doorway is a page full of links, called the Farm and Ranch Links of links, each link leading to the User to a different content than it looks. This is negative for the User who is frustrated when accessing this page and the browser with anger that showed this result to him, and for the browser, in Google’s case for example, that every day becomes more demanding with the content always thinking in retain its User. When Google finds this tactic not only Doorways are penalized and the sites that link to them.

Example Doorway to Link Farms:
Imagine a (site X) with 300 pages and each one linked to the (site Y). We will then have 300 external links to the (Y site) will result in a good relevance in terms of PageRank and therefore a comfortable position in search engines. The (site Y) which also has 300 internal links do the same thing and that every new page created new links are becoming a Farm Links. These techniques are already well known by search engines, especially BING & Google, therefore, should be avoided.
Another method widely used among bloggers, especially, is the exchange of links.
Be very careful with the exchange of links to not fall into the same trap of links farms.
Please exchange links with sites that have content relevant to the content of this site is related to its use and only the index page for that or a unique page for exchanging links. Use the techniques of building links (link building) on social networks, a member of directories, social bookmarking and so on.

Do not use hidden text. Example
• Use of text on a white on white background
• Including text behind an image
• Using CSS to hide text
• Setting the font size to 0 (zero)

Do not hidden links.

Example
The hidden links are those that can be crawled by Googlebot, but are unreadable to humans because:

• The link consists of hidden text (for example, with the source text and the background the same color)
• The CSS has been used to create hyperlinks the size of a pixel
• The link uses a small character to keep it hidden – for example, a hyphen in the middle of a paragraph”

(…) If we feel that your site has text and hidden links purposefully deceptive, it can be removed from Google’s index and will not appear in the pages of search results. In assessing the existence of hidden text and links in your site search items that are not visible to visitors of the page. texts are present exclusively or links to search engines and not for visitors ?”(…)
See more at: http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=66353 
 

Any SEO technique used for manipulation of results is considered Black Hat SEO and consequently the site that uses it will be excluded from the Google index.

Learn more about the SEO techniques which should be applied.