Quick guide to some important tags for SEO

Rel=Nofollow

For use in a link as an attribute.

<a href="register.php" rel="nofollow">register</a>

What does Google do with nofollowed links? Google does not follow them and they do not transfer PageRank or anchor text across these links. When Google sees nofollow in a link, it will not follow through to that page, it will not transfer PageRank and it will not take anchor text into consideration.

More on this here – http://www.google.com/support/webmasters/bin/answer.py?answer=96569″

No Follow

For use in the page header as a META tag.

<meta name="robots" content="nofollow"> 

Remember that the NOFOLLOW only applies to links on the current page. A robot might still find the same links on some other page without a NOFOLLOW (even on other site), and could still find the page you did not want to be found.

Don’t confuse this NOFOLLOW with the rel=”nofollow” link attribute.)

More on this here – http://www.robotstxt.org/meta.html

Canonical tag

You have a page at www.widgets.com/blue-widgets/ – this is your main page for Blue Widgets. But you have a few different paths to the same content i.e.

1. www.widgets.com/products/blue-widgets/
2. www.widgets.com/blue-widgets/products?category=widget&color=blue

You can indicate to Google which page is the one they should focus on and not see the other pages as duplicate content.

Put this link on all the pages (in the header) that are NOT the main page for Blue Widgets:

<link rel="canonical" href="http://www.widgets.com/blue-widgets"/>

No Index

To prevent all robots from indexing a page on your site, place the following meta tag into the section of your page:

<meta name="robots" content="noindex"> 

To allow other robots to index the page on your site, preventing only Google’s robots from indexing the page:

<meta name="googlebot" content="noindex"> 

When Google sees the noindex meta tag on a page they will not display the page in the search results, even if other pages link to it. Not all search engines support this tag so, a link to the page can still appear in the search results.

This from Google:
Note that because we have to crawl your page in order to see the noindex meta tag, there’s a small chance that Googlebot won’t see and respect the noindex meta tag. If your page is still appearing in results, it’s probably because we haven’t crawled your site since you added the tag. (Also, if you’ve used your robots.txt file to block this page, we won’t be able to see the tag either.)
If the content is currently in our index, we will remove it after the next time we crawl it. To expedite removal, use the URL removal request tool in Google Webmaster Tools.

More on this here – http://www.google.com/support/webmasters/bin/answer.py?answer=93710