Search Engines use a number of factors to calculate how relevant a website is to a particular search term. Unfortunately, they don't publish these and the SEO profession uses analytical techniques to determine which factors are most important.
According to Seomoz's ranking factors which has polled the opinions of the world's leading SEO professionals, 4 out of the top 5 ranking factors involve links to your website from other websites.
Why is link building important?
Google assesses the number of incoming links, the number of different websites these links come from, the text used in links and whether these links come from trusted sources to help rank its results. If you want to improve your listings on Google then you need to improve the number of incoming links that match these criteria. Lets consider the main link building concepts.
Link popularity is a measure of the quantity and quality of links that go to a website. If you think of a link to your website from another website as a vote then you increase your link popularity by getting more votes. However, not all web pages are equal. Pages with a lot of votes (link popularity) themselves are worth more than pages with little link popularity (votes). This concept is designed to filter out less important pages and reward popular or trending pages.
Link popularity also mirrors human behaviour, as good content will attract a good number of incoming links where as poor content won't. Getting more quality links to your website will increase your page's link popularity. A word of caution; quality is more important than quantity. Google uses a lot of smart calculation to filter out low quality or spammy pages & links to stop people manipulating its results (see Domain Trust below).
As well as Link Popularity, Google also looks at the number of domains linking to your site as a measure of Link Diversity. 10 links from 10 different websites have more value than 100 links from the same website. Link Diversity is quite difficult to manipulate as it might be easy to create a website with 10,000 pages which all link to your site but its very difficult (and expensive) to create 10,000 websites that all link to your site.
To summarise, as well as gaining a large number of links from quality sources, we need to make sure that these come from a diverse range of domain names.
On the subject of manipulating results we need to consider a domain's Trust. This is a measure of the chance that this domain contains spammy or manipulative content designed to fool the search engines. If we were to manually veto the web, then trusted websites are those that contain no spam. Untrusted websites are those that only contain spammy content.
Obviously even Google would struggle to manually veto the entire web so Search Engines count the number of links you have to follow to get from a fully trusted website to your website to calculate your site's trust. If your website is closely linked from well trusted sites then it's likely that you are running a trusted website. Likewise, if you have lots of links from Low Trust, spammy websites then it is likely that you are running a low trust, spammy site yourself.
Google also looks at domain registration information to connect websites that although they don't directly link to each other may be run by the same business or from the same address. If a business is running several spammy sites already then there is a good chance that future websites launched by that business are involved in spammy, manipulative practices.
So to bring this back to Link Building; as well as having a high number of quality links from a diverse range of domain names, we need to ensure that the pages that link to us come from trusted domains to avoid being penalised as a spammer.
The final (and possibly the most important) link concept I would like to cover is anchor text. This is the text that is inside the link i.e. the hypertext you click on to go to another page. If you use 'click here' for the anchor text of your links then Google will associate the destination page with the keyphrase 'click here'. You can see this in action by putting in a Google.com search for click here which returns the number one result as being the Adobe Acrobat Reader. Why? Well thousands of websites use PDF documents and many include text like:
This document requires the Acrobat Reader, click here to download it.
This associates the download Acrobat Reader page with the keyphrase 'click here'. Not very useful I know but the same applies to your website. If you are optimising for a particular keyphrase then it is very important to consider the anchor text of your incoming links. Most links will use your business name or website domain name so some consideration should be given to including a target keyphrase in your business / website name to take advantage of this effect. If you are a financial services company then xyzservices.com might not be as good a domain name (from a SEO point of view) as xyzfinancialservices.com if you are optimizing for 'financial services' as a keyphrase.
Whilst we are on the subject of anchor text, this also can be manipulated for comical or political purposes in a practice which has become known as Google Bombing. In 2006, if you entered a Google search for 'miserable failure' the top result would be a biography of George W. Bush. There have been numerous other examples which have been gradually filtered out by Google updating their algorithms but all of this clearly demonstrates the importance of link anchor text in search rankings.
We have been talking about a number of quite abstract concepts but now we need to bring this back into the real world. All these linking metrics are designed to mimic human behaviour on the Internet. If a webpage or site has something interesting to say or offer then people will naturally link to this site and say 'Hey, this is important, you should take a look'. How they do this is changing over time (and will be a subject we will cover in another post) but the most important thing to remember it is the visitors that are important and not the Search Engines.
If your link building efforts are just for the Search Engines then you might have some short term gains but you wont have as much long term success as those who concentrate on what visitors are looking for.
In this post we have introduced the topic of link building and covered some of its most important concepts. In the next post we will highlight some of the top link building strategies and give ideas on how you can implement this on your website."