What makes a good link from an SEO perspective?
The external links that point to your site play an important role in where your site appears in the search engine results. You can think of it this way: every link from a relevant external site that points to your site counts as a 'vote of confidence' in Google's eyes. In fact, it is agreed by search engine optimisation professionals that 4 out of the top 5 factors affecting your Google ranking are about external links. But when it comes to search engines, not all links are created equal. So what makes a good link from an SEO perspective? If you have partners or customers who are willing to link to you, how exactly is the best way for them to do it, in order to give you the maximum benefit? Here's my take on this, hopefully explained in a way that anyone (even non-techies) can understand. Let's start from the worst moving up to the best...
DREADFUL: Image Link, no ALT tag, meaningless image name
SEO-usefulness: 0.5 (out of 5)
This link is not really doing you any favours. OK, it's a link, but it's not working very hard as far as SEO is concerned. Google - like all other search engines - cannot 'see' images. So even though this link says SEOMoz, the words are part of an image. You can tell by trying to highlight it with your cursor: words that are images do not highlight as text. Therefore, Google can't really tell what this is.
This image has no ALT tag, either. An ALT tag is a little bit of code that tells visually impaired people (and search engines) what the image is. So for a photo of a green apple, the ALT tag might say "green apple". You can tell if an image has an ALT tag by hovering your mouse over an image. If there's an ALT tag, the text will popup (some people call this 'tooltips').
Finally, the filename of this image has nothing to do with the website where it is pointing. The filename is "logo135b.jpg". Unless someone goes onto Google and types "logo135b", this is no help whatsoever. In fact, I encourage you to go to Google now and type logo135b in the search box. Does this give you any clues as to how image names can help with SEO?
POOR: Image Link, with ALT tag, meaningful image name
SEO-usefulness: 1 (out of 5)
I see this as the bare minimum when it comes to a link that is useful from an SEO perspective. Even though Google still can't 'see' the image, it can read the text in the ALT tag, and that text contains the brand name of the website it is pointing to (hover your mouse over the image to see). Furthermore, the filename of the image is "seomoz.png" - which also contains the brand name of the website it is pointing to. Luckily, in this case, the brand name and website URL have something to do with the services that the company offers. But if your company name is something like "Deep Blue" and you sell shoes, you're not so lucky. There's nothing in your brand or URL that tells Google - or human visitors - that deepblue.com sells shoes. So the usefulness of any links that mention the words "Deep Blue" are only helpful to you from a branding perspective. We'll expand on this next...
FAIR: Image Link, keywords in ALT tag, keywords in image name
SEO-usefulness: 2 (out of 5)
If you simply must use an image link (for branding purposes/advertising), this is the way to do it. The ALT tag here is "search engine optimization resources" - and this contains keywords that people might use when searching on Google for such a site. The filename of the actual image itself is "search-engine-resources.jpg" - again, this contains keywords. Including keywords as part of the link is absolutely critical: Google sees this as saying "Hey, look over there on that site if you are interested in search engine optimization resources". Naturally, this only works if the site you are pointing to actually contains search engine optimization resources. Google isn't stupid.
You can further improve the usefulness of the image link by adding a text link directly under or next to it, something like the below...
MIDDLING: Text Link, keywords near the link text
SEOmoz.org - a good place to look for search engine optimization resources
... if you are interested in reading more about search engine optimization, you might look here.
SEO-usefulness: 2-3 (out of 5)
Now we're getting somewhere. Google likes text, and text-based links. More importantly, remember that the success of Google is based on TRUST and RELEVANCE. So this is what Google uses when evaluating links. We'll get to the TRUST part in a minute, but as for RELEVANCE, this is how it works: Google sees a link on a page, and if there's nothing in the link itself to give clues about what the linked site is about, Google then looks at the text in the immediate vicinity to find out what the link is pointing to. It is common sense, really. The words around the link give context, and indicate that the link is relevant to that topic.
Now, ordinarily a link with a generic word like "here" (above) is a bit of a wasted opportunity. But luckily, this one has been partially rescued by putting some relevant keywords in the link title (hover your mouse over it to see). However, it's still not as good as the link above it, and nowhere near as good as the following...
GOOD: Text Link, keywords in the link text
SEO-usefulness: 3 (out of 5)
Even better: use the keywords in the link text itself. This is a clear signpost to Google indicating that the linked site is all about the topic of search engine optimization. Equally importantly, it's a clear signpost to site visitors that when they click on that link, they will find search engine optimization resources. This is almost as good as it gets in terms of relevance...
I say almost because relevance is not just about the words immediately in and around the link. What is on the rest of the page?
BETTER: Text Link, keywords in the link text, on a page about the relevant topic
As above, but in an article about SEO
SEO-usefulness: 4 (out of 5)
Naturally, if the rest of the page is about web design, and there's a link to an SEO site stuck on, then this is not as good in terms of relevance as a link on a page/article purely dedicated to SEO. So if you think about my own blog (what you are reading right now), the links I have used as examples on this page are exactly what we are talking about: the text links above are a 4 out of 5 in SEO-usefulness for SEOmoz because this entire article is about SEO.
Think about how most links occur on static websites: more often than not, they are lumped into a page called "Useful Links" or something. Usually there is nothing on that page that gives any relevant info about those links. Now think about how most links occur on blogs: usually they are sprinkled throughout the text in an article about a relevant topic. With this in mind, is it any wonder why blog pages often appear high in search engine results*?
But wait, there's more...
BEST: Text Link, keywords in the link text, on a page about the relevant topic, on a trusted site
As above, in an article about SEO, on a site trusted to be about SEO
SEO-usefulness: 5 (out of 5)
This is the Holy Grail of value when it comes to external links and SEO. Remember, I said we'd talk about TRUST later, so here we are. Google wants to give customers the best possible results, so it makes sense that a link from a trusted, relevant website is best. You would do the same: you'd trust a mechanic's car recommendation more than you would trust a baker's. At the beginning of this article, I said a link is like a vote of confidence, so a vote from a website that is well-established in its niche area is better than a vote from a site which isn't.
If we look at the example of the SEOMoz links, if all other factors are equal, a link from my blog is not going to be nearly as valuable as a link from a site like Search Engine Watch. Search Engine Watch is all about SEO, it's been online for many years, it has lots of relevant SEO links pointing to it, and it ranks high in search engines itself. In this sense, Google trusts Search Engine Watch more than it trusts me. Which is fine by me - I trust Search Engine Watch more, too!
So there we have it, the good, the bad and the ugly when it comes to inbound links and search engines. Now you can make better use of adverts, partner links and hopefully make better links yourself, too.