Domain Authority is an estimate of how strongly rated your domain is estimated to be. It is based on the strength of the other domains that are linking to your website. If no domains are linking to your website, your domain authority will be very low. If several websites with very high domain authority have linked to your website, then your domain authority will be strong. The higher your domain authority, the better your Google and Bing rankings.

This is why YouTube, Amazon, Wikipedia, Facebook, University and other high traffic resource websites dominate the Google search results. They all have domain authority in the 90s. If you can get links from those sites, and back link to those sites, that domain authority gets passed to your website and it will climb in the rankings.

It is very important to link back to an internal page of a high domain authority site that is linking to your domain, not their home page. Make sure that the page that you are linking to matches the content of the page that you are linking from.  You also want to try to avoid linking directly back to the page that the site is linking to you from. It is not a huge problem if you do this occasionally.

If numerous pages on your website link back to the same pages within the various domains that link to your website, that will get Googles’ attention as an unnatural linking profile and earn you a penalty, sometimes a very severe penalty if you have done this too much.  If you keep linking to the home pages of the various domains, it will also look like an unnatural linking profile and you will wind up with a penalty instead of bonus traffic.

I know that I have said this already, but this is critical; to avoid getting a penalty, link back to different internal pages on the other domains, that have similar content to the page you are linking from.  This will show Google and Bing that you have put a lot of thought into your content and that you are trying to deliver a great experience for your readers.

If you are not aware of this yet, whenever you link to an outside website – always make sure that the link opens in a new browser window.  That way when the reader closes the new window, your site is still open in the background.  You may have people continue reading your website and it will also show that people have spent a lot of time on your website even though they were reading something in a new window.

There are exceptions to how you link to pages on some domains:

Passing on YouTube domain authority;

If you have your own YouTube page and you are placing your YouTube videos on your website from this page, you should be OK. I would suggest that you also link to other YouTube videos from other, relevant YouTube pages just to increase the diversity of links.  If you write a new page on a topic, find a YouTube video that relates to that topic and use the YouTube embed link to embed that video on your website as part of your article.

Passing on FaceBook domain authority;

If you have a Facebook page, you will be posting your content updates on your Facebook page and linking back to your website. Everyone does this and it does give you a slight boost. What you also want to do is find the Facebook pages that are similar your website and if they have good, relevant content, write a new article on your domain and use link it to their Facebook page, or the post that they wrote.

http://www.opensiteexplorer.org
This is a free tool, but it is limited in how much information you can see. It only lists the Domain Authority for the top 5 websites that are linking to your website. It is still useful though. You can scroll down the list and look at all the sites that have linked to your site and you may spot some sites that you recognize. These are the sites that you will want to link back to.

Look for .org websites and then put the domain into the Site Explorer and check their domain authority. If it is legitimate .org website, it will have a domain authority greater than 50. The higher the better. .gov websites are great, so are recognized domain names, but check them in the site explorer first to make sure they are real sites and not someone who set up a copycat domain using a name variation. .org websites can be bought by anyone, so make sure you check them. You can just as easily drop your domain authority by lining to bad websites. Google has a neat penalty waiting for you for this whenever they run the Penguin algorithm.

The best way to back link to a high domain authority website that has linked to your website is to add a link back to their website through new content. You could find a page on their domain that is relevant to the content on your domain and write 1000 to 1500 about the relevancy and then link to that internal page in their domain within your new content.

http://moz.com/learn/seo/domain-authority
This explains how Domain Authority is calculated. Because Domain Authority is hard to fake, it is becoming a very valuable indicator of how well your site is viewed by other sites. If a lot of sites that rank high for Domain Authority are linking to your website, it probably means that your site is being viewed as a high quality site by other high quality sites.

Two sites that you want to try to get into. These both have very high domain authority and all the sites are reviewed by people, so Google and Bing put a lot of credibility on the domains that are listed in these directories.

http://www.dmoz.org
DMOZ is also known as Open Directory. It is made up of websites that are reviewed by people, so everything in this directory is real and legitimate.  It can take sometime before your site shows up in the directory, because the people reviewing the sites are all volunteers and they each may have numerous categories to oversee.

http://dir.yahoo.com
Yahoo Directory is similar to DMOZ in that a person reviews your domain and if they feel that it has good content and it is relevant to that category, they will include it in their directory. The Yahoo directory results not the same as Yahoo search results.  Yahoo search results will not show up in the Yahoo Directory, but Yahoo Directory results will show up in Yahoo Search results.  US users have to pay annually for inclusion in the Yahoo index. Canadian and foreign users did not have to pay for inclusion, but this may have also changed.

Google Places a lot of weight on being in the Yahoo Directory.  It used to place a lot of weight on being in the Open Directory, but over the years Open Directory has become less relevant, but still very important.  It is still a very good idea to try and get your site into Open Directory and Google will still place a lot of weight on this, because your site was reviewed by a human editor, so they know that it is a real, and legitimate, website.  This will give your domain ranking a boost, even if you cannot get into the Yahoo Directory.