Cloaking is a technique that is used to deliver one page to a search engine for indexing while serving an entirely different page to human visitors (GreatNexus.com). This technique enables SEOs to optimize cloaked pages for the search engines, without needing to compromise on usability or aesthetic elements. Search engines do not always penalize sites that cloak, but they do often detect cloaking and discourage the use of it. (Google a, Yahoo! a.)
Link farming, by definition, is the process of exchanging reciprocal links with web sites for the sole purpose of increasing link popularity of a site (Webopedia a). Exchanging links with sites that are irrelevant to users and/or provide no useful content, is not only a waste of time, but may also harm rankings in the search engines.
Duplicate domains refers to having duplicate content on two or more separate domains. By having the exact same or slightly altered content under different domain names, a company or individual may be able to dominate particular keywords by having more than one site ranking in searches. The current algorithms often fail to recognize such spam, but their usage is highly discouraged.
Redirects are auto-forwards that are done either on the browser or on the server level. For example, the URL of a page has changed from http://www.example.com/example.htm to http://www.example.com/example/index.php. When the change is made, there could be links pointing to the old page. People that click on those links would get a “404 page not found” error message unless a redirect to the new URL was used. There are three redirects that are commonly used on websites, 301 permanent redirect, 302 temporary redirect and meta refresh. The 301 redirect is done on the server level by modifying a file called .htaccess. The 301 redirects are completely safe from SEO standpoint and should be used whenever a URL has permanently changed.
The problems with redirects arise when 302 redirects and meta refreshes are used. The 302 redirects are done on the server level, while meta refreshes are done on the browser level. The biggest concern on the use of these temporary redirect types is that they are being exploited on a large scale to hijack web pages in a way that the hijacker’s page replaces the original page in the search results, causing the original page to vanish from the SERPs ( Clsc.net ). At the time of writing, Yahoo! is reportedly the only search engine that has been able to fix this bug. Using these types of redirects is therefore highly discouraged, as search engines may assume that the redirect is intended to hijack or spam the search engines, even if it is not.
Keyword stuffing is an optimization technique that is used by both white hats and black hats. Keyword stuffing is about repeating certain keywords and phrases. As keywords are the main element upon which optimizers build their campaigns, every SEO uses keyword stuffing to some extent. Keyword stuffing has a negative ring to it, as optimizer often think of it as repeating a keyword excessively throughout a page. In the end, using keywords conservatively will actually be of more benefit to a site than the unnecessary stuffing. Rule of thumb in keyword usage in text is that as long as the text “flows”, there should not be a problem with the search engines.
Hidden text or invisible text refers to content that is visible to the search engines but invisible to the users. One of the most common example of hidden text is setting the font of the page to be of the same colour as the background colour. Search engines will see the keyword-rich text, while users will never know of its existence. At the early stages of search engine optimization, webmasters used meta-tags to let search engines know which keywords are relevant to their web pages and what description search engines should used in the search results. As webmasters started abusing meta-tags by inserting irrelevant keywords and phrases, search engines needed to find other ways of ranking and determining the content of the web pages. Visitors cannot see meta-tags, so they are an example of hidden text.
Search engines caught up with hidden text a long time ago, but optimizers come up with new ways to hide content and fool search engines. Nevertheless, stuffing hidden content with keywords is a very spammy technique that companies should not use under any circumstances.