Hi,
Sorry for the delay in answering your questions.
Google considers almost every incoming link to a site as a "vote" for that site. They pay careful attention to the linking text, among other factors. Due to this, many unscrupulous webmasters surf the internet spamming comments on blogs and other locations that allow un-moderated posts.
No follow tags are notifications to search engine spiders to not crawl certain links found on your site. As a blog owner, I can configure my site to automatically attach nofollow tags to the links left in comments so that such spammers don't benefit from my site linking to theirs.
An example of a nofollow link is:
Code:
Visit my <a href="http://www.example.com/" rel="nofollow">discount pharmaceuticals</a> site.
Google has a nice blog post that discusses
nofollow tags in detail and their actions to limit comment spam.