In the early days of the Internet, webmasters felt that the more a key term was used on a Web page, the better the site would rank for that term. The belief was that search engines would find the site more relevant and rank the pages higher. For a while this tactic worked. Today Google frowns upon the practice of "keyword stuffing" and may actually penalize your site and remove your site from the Google index if they suspect you are using this technique. Google defines "keyword stuffing" as: "the practice of loading a webpage with keywords in an attempt to manipulate a site's ranking in Google's search results."
There are several forms of keyword stuffing that should be avoided.
Invisible Text: Text that is on a page that is the same color as the page or background (i.e. white text on a white background) where the viewer cannot see it, but the search engines can. Many spammers would repeat the keywords over and over. The thought was if one use of a keyword was good ten was better. This was done in hopes of increasing the relevance of the page without affecting how the Web page looked.
Irrelevant Keywords: Adding non-relevant terms with a high activity count into the Meta tags with the hope of attracting more traffic to the site and raising the site's rankings. Keywords used on the site need to be relevant to the topic of the page and actually have to be used within the content on the page.
Hidden Text and Links: Hiding text behind images or using CSS to create links or text that are not viewable to the user by using a <div> or absolute positioning. With a <div> you can specify the browser to hide a specific portion of text and with absolute positioning you can specify the text to be positioned off the screen so it isn't viewable for the user.
Keyword Stuffing: Adding keyword terms over and over again in content, Meta tags, Alt Attributes and comment tags. The intent is to make the search engines believe that the page is relevant for the keyword term and thus inflate its ranking. This can adversely affect pages because, if caught, search engines will penalize a keyword stuffed site or completely remove it from the index. We always recommend having your most targeted keywords appear in the Title tag, Meta Description tag, Meta Keywords tag, Heading tag and of course within the content. As a general rule, we recommend not repeating a keyword more than once in the Title tag, twice in the Description tag and not more than four times in the Meta Keywords tag. Remember, search engines can penalize your site for keyword stuffing. Write for the user not the search engines. The content should read naturally, not feel or sound forced.
Duplicate Tagging: Using the same or generic Title tag and Description tag repeatedly on pages across the site can be harmful. Your pages can be filtered and in some instances a significant number of pages can be penalized and removed from the index. This repetition is not always intentional as your CMS system might automatically generate the Title tag and Meta Description tag. Your best option is to write a unique and relevant Title and Description for each page. Google has a new feature within the Webmaster Tools that can help identify pages with duplicate Titles, Meta Descriptions and non-indexable content.
Keyword stuffing does not provide any value to your site and other sites won't want to link to you. As mentioned previously, the Web page should read naturally. In the long run, keyword stuffing will just cause more work for you on your site as you go back to correct instances of keyword stuffing. Otherwise, the search engines will eventually filter your pages and possibly drop them from the index. Keywords should be used appropriately and evenly throughout the Meta tags, linking, navigation and the headers and footers on a Web page.