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What you are likely seeing is blogs rather than regular websites. Blogging platforms like WordPress allow authors to place their blogs into categories and tags related to their posts and usually these categories and tags are displayed at the bottom of the post. Here’s an example:
The above image shows the last part of the post. The seemingly random words in there are tags. And the word after “Filed Under” is the category the post was placed in (in this case “seo”). Is this like what you’re seeing?
This is not to be confused with spamming keywords on a web page (although blog tags can be used for this as well, though that isn’t their purpose). Keyword stuffing is a black hat method of SEO that, when caught out, is likely to get your website penalized. Tags and categories are meant to relate to the blog post – many sites which aren’t even blogs just put random keywords in some part of the page to trick search engines into ranking them higher for those keywords. Here’s an example by Matt Cutts of Google:
http://www.mattcutts.com/blog/avoid-keyword-stuffing/