The internet is a network of networks, and each of them are connected through links. When quality matters, not all links are created equal.
A link consists of three elements: trust, diversity and relevance. A link that is pointing to your website is counted as a vote. But you should always consider the quality of the link because without this concern, you're just making your site valueless.
On the early days of the internet and SEO, search engines were not as good at determining the quality of links. With the introduction of Google's PageRank among many things, spamdexing has been reduced.
Search engines are now becoming more advanced, and they're able to expand the link-related signals to go beyond mere numbers. To determine the quality of a link, search engines can look at a number of factors, which can all combine to conclude an indicator of quality.
With this, they can determine whether a link is genuine, given or a spam.
Trusted Links
Ideally, links are those that are given from other websites to you by trust. Some may refer this as an authority. When Google came up with PageRank, it's creating a way to objectively measure every link it finds on the web. The more PageRank a page has, the more it is trusted, and the more likely it will rank higher in its SERP.
There is another concept that can make a links trusted. It's called TrustRank.
PageRank is one popular method to improve ranking through linking schemes. TrustRank seeks to combat web spam by filtering the web based on reliability. The work is done by selecting a small number of pages to be evaluated manually.
TrustRank differs from PageRank as it aims to eliminate spammer. Taken from the Combating Web Spam with TrustRank by researchers Zoltan Gyongyi and Hector Garcia-Molina of Stanford University and Jan Pedersen of Yahoo!, TrustRank is indeed able to identify spam effectively but not necessarily efficiently since manual review of the internet is impractical.
Diverse Links
Diversity here means getting links from different types of domains, not multiple links on a domain over and over again. Diversity in links can be described as: diversity of linking domains and diversity of link type.
To bring a robust link profile, it's essential to understand those two diversity in link building. If you links are coming from many domain names, your links are diverse. But if you links are coming from a single site, they aren't diverse and aren't natural.
Search engines like to see diversity. And because diversity comes naturally, this signals search engine that the site indeed has an authority about a specific subject/topic.
Relevant Links
Links should come from relevant place, created with relevant anchor.
If a link comes from a certain topic, the link should forward the visitor to a similar page talking about similar topic. This makes the two pages relevant, thus making the links also relevant to search engines. Anchor text on the other hand, allows Google and other search engines to discover the possible topic without having to crawl it. Anchor text is like a signal, and with relevant anchor against the page, it signals a strong relationship between the two.
Anchor text can include keywords if necessary. Unless you're overdoing it, keyword-focused anchor texts despite risky, can be used to create relevancy among pages and links.
What Affects The Quality Of A Link
Your links and your web traffic are friends with benefits. Not every links are created equal, and so do pages. Since every page is made differently, they can also affect the quality of the links put into it.
Since Google and other search engines are getting smarter, they're not anymore looking at the raw number of links pointing to a website. They're looking at other numerous factors to determine the quality of a link, and how much value should they pass to a targeted page.
Number of Links
One page can include hundreds or even thousands of links. If you link is among them, the chances are your link isn't at all valuable. This apply to both search engines and visitors.
Pages that have too many links pointing out of their site, will have a higher bounce rate. And those link won't also be particularly useful because the page isn't giving a good user experience. There is also the chance that the page is only created to home those links and never have real content on them. This is a signal to search engines, saying that the page, and its links, are indeed not valuable.
From a PageRank point-of-view, the higher the number of outgoing links inside a page, the lesser the value each of them represents. Despite being debatable, outgoing links that are too many aren't going to improve user experience even a bit. And both search engines and human visitors are putting high value on pages that give great experience.
Beside seeing how many links you have pointing to and from your website, you should also consider to see whether those links are coming from related websites, or even the possibility of links coming from banned websites or penalized website. Despite those links can't hurt you that much, the higher the number the higher those links can pose bad reputation to your site.
Links should come from websites that are reputable and respected, apposed to spammy ones. The concept here is the idea of having one high-quality link to be a lot more ideal than hundreds coming from websites which have thin contents worth next to nothing.
Quite often, pages on the web with high numbers of outgoing links will have lower editorial standards. And the opposite, pages that have high-quality links will have high editorial standards.
You definitely need to avoid having your website's link shown inside low-quality pages. If your links are inside those pages, this signals Google that your link building strategy is aiming on low-quality backlinks, and could be spams. You need to take a closer look to see whether or not these links are hurting you.
The next is the domain's age. Despite not strictly a factor, the older a domain and a website, the higher chances that they have a higher number of contents and links pointing back to them. The more the domain has these two, the more it's valued.
Related: Administrative Relationship, And Why Google Thinks It Needs To Know What Websites You Control
Elements of a Link
Links that are more "reasonable" to visitors are the ones that have more value. But what makes them great is their elements giving them certain value. This can include:
- Position (header, sidebar, footer, etc.).
- Font size.
- Text/image/media/Flash.
- Anchor text.
- Etc..
To signal search engines saying that your link is valuable, you should avoid the link to go through redirects, prevent it from being blocked by the robots.txt file, having a page with viruses/malware, using characters that Google can't understand, contains extra parameters at the end of the URL, and so forth.
These simple signals can alter the way search engines handle the link. Search engines can either follow the link or not, or chooses to pass PageRank across it or not. But if they think that the link, and the page the link is pointing to, can harm visitors, search engines may penalize or even block that link to protect their users.
Search engines may even decide to take those pages out of its index altogether or make them very hard to find..
Further reading: The Reasons Why Your Backlinks Aren't Helping Your Search Engine Rank