You can confer some of your site's reputation to another site when your site links to it. Sometimes users can take advantage of this by adding links to their own site in your comment sections or message boards. Or sometimes you might mention a site in a negative way and don't want to confer any of your reputation upon it. For example, imagine that you're writing a blog post on the topic of comment spamming and you want to call out a site that recently comment spammed your blog. You want to warn others of the site, so you include the link to it in your content; however, you certainly don't want to give the site some of your reputation from your link. This would be a good time to use nofollow.
Some search engines have also reached out to the SEO industry, and are frequent sponsors and guests at SEO conferences, webchats, and seminars. Major search engines provide information and guidelines to help with website optimization. Google has a Sitemaps program to help webmasters learn if Google is having any problems indexing their website and also provides data on Google traffic to the website. Bing Webmaster Tools provides a way for webmasters to submit a sitemap and web feeds, allows users to determine the "crawl rate", and track the web pages index status.
Off page SEO: This type of SEO focuses on all of the activity that takes place "off the page" when looking to optimize your website. "What activity not on my own website could affect my ranking?" You might ask. The answer is inbound links, also known as backlinks. The number of publishers that link to you, and the relative "authority" of those publishers, affect how highly you rank for the keywords you care about. By networking with other publishers, writing guest posts on these websites (and linking back to your website), and generating external attention, you can earn the backlinks you need to move your website up on all the right SERPs.