This is a html element which allows to browsers that recognise the <script> tag, but may not necessarily support the script in it. Whatever is displayed within the <noscript> tag will not be displayed by the browser. This allows for programs like traffic counters to hide their links and not display the counter information on a web page. For example statcounter employ a <noscript> tag within their tracking code,

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<noscript><div class="statcounter"><a class="statcounter" href="http://www.statcounter.com/"><img class="statcounter" src="http://c29.statcounter.com/hghghjfff/0/ghfjjjjj56375n/" alt="hit counter script" /></a></div></noscript>

The use of the tag as shown above would be classed as a legitimate use of it.

However, the <noscript> tag has been abused by some webmasters. Until recently search engines including Google would follow a hundred or so links placed in this tag. Until recently I have noticed Google still displaying information located in this tag in their search results. So it's an old trick, where webmasters can stuff 'x' amount of keywords and links in a <noscript> tag, Google will record this information as genuine. But in reality it's spam and hidden from a user. With the advancements in Google's crawling techniques this may not be the case for long.