Whenever I'm asked about what I do for a living, I say something like this: "you know those pieces of text that you can click on inside of a webpage, the ones that take you somewhere else? I place those."
Blank stare. Sometimes they respond with, "OK, but why?" That's a damn good question. The "why" behind the existence of links has been a bit more absent than it should be, especially for people who are new to the field.
Why Do Links Matter?
Hyperlinks were the main method of building the Internet and connecting sites through HTML, allowing people and bots to move around and find what they needed. They were like any other citations, methods of getting additional information by going somewhere else.
Contrary to popular belief, Al Gore didn't invent the hyperlink. The term itself was first used in the 1960s, before most of you were born.
In 1998 there was the first on-paper mention of PageRank, just before Larry Page and Sergei Brin actually founded Google. The theory behind PageRank became part of the basis of the Google algorithm, and continues to be so today.
To greatly simplify the concept, PageRank is a popularity contest wherein the pages with the most support (via inbound links) behind them should be viewed as the most important ones. You could increase a page's importance simply by building as many links as possible to it.
As anyone who deals with SEO knows though, it's a lot trickier than that.
Not All Links Are of Equal Importance
A link from the homepage of a powerful site like the BBC will be of a higher quality than a link from the links page of your high school's blog.
If a competitor that ranked above you in the SERPs had 100 more links than you, you couldn't just go grab 101 links and rank above him. Some links are simply more valuable than others, particularly links from authoritative sites (like respected news sites) and links from .edu and .gov domains.
Like every other SEO tactic, this was abused, differing opinions abounded, and everyone tried to nail down the exact science of it.
In 2005, the nofollow link attribute came along and ruined all our fun. No longer could we throw tons of links at sites in order to make them rank. That can still work as you'll see at times, but quick wins with links aren't as plentiful as they were pre-nofollow.
In 2009, PageRank was removed from Google's Webmaster Tools, mainly due to the fact that people didn't really understand that the number they saw wasn't a true representation of their sites's importance (and was updated about as frequently as your grandma's hairstyle.)
Note: there have been some updates to the original PageRank patent, which Bill Slawski covers in detail here.
The PrePageRank World
What did we do before we had that pesky little toolbar indicator? Without that one commonly misunderstood metric to constantly monitor and agonize about, we used rankings and traffic as an indicator of our performance.
We could also rank a site without links, just by keyword stuffing (cramming keywords into my tags and content to the extent that 50 percent of my words were that exact keyword, for example) and cloaking (figuring out how to send search engine spiders to one place where I keyword-stuffed while showing users a nice, pretty page). Those were the good old days when you could get a link on a site and not get cussed out by your client because they wanted all PR 4s and up and you, stupidly, got a link on a new but very relevant and well-trafficked PR 0 site.
We still knew that links were important. They just didn't make us crazy.
Link exchanges were very big. Having a page just devoted to outgoing links was huge. It was a softer, gentler time when link building as we know it today was innocent. The only people that I knew who built links were generalist SEOs, and looking back now, it's easy to see that we did it badly by today's standards.
There's a point that gets lost a lot, one that makes it obvious that actual PageRank and visible PageRank are two very different things.
The PageRank that we can see represented in the bar, a number, from a PageRank checker, etc., is updated infrequently and isn't the actual PageRank that Google assigns to your site. The actual PageRank calculation, if shown here, would make all of our heads spin. Let's just say that it's a lot more complicated than a number from 0 to 10.
This is what you do see (and sometimes confuse with actual PageRank.) Toolbar PageRank is one of many factors in how your site will rank but its importance is way overblown and oversimplified. You will see sites with a Toolbar PageRank of 1 outranking sites with a Toolbar PageRank of 5, due to various other considerations (like social signals, for example.)
PageRank Sculpting and Link Juice
Now here is where things get particularly interesting to me. Pages have their own specific PageRank (both actual and toolbar) and through linking elsewhere, they can send link juice in the same way that they receive it.
If a page has 10 outgoing links on it and none are nofollowed, each page linked to should receive one-tenth of that page's link juice. If five links are nofollowed and five are not, each of those five followed links should receive 20 percent of that page's link juice and the five nofollowed links should receive none of it.
Due to this idea, people began to experiment with manipulation. (Can you imagine SEOs manipulating anything?) We nofollowed certain links that went to other site pages, ones that weren't quite as important as the others but ones that we did link to in the navigation. That seemed OK.
Later, like with almost everything else, it got complicated. I won't bore you with the details here. Suffice it to say it's not a widely recommended practice anymore. Some still do it, some don't, but controlling link juice didn't work as we hoped it would. You'd think we would all learn our lessons but no, no we never do.
So Why Do Links Matter Today?
Oddly enough, they matter for the same reasons that they have always mattered: they send traffic by making connections and yes, they are still a large part of ranking. I don't see that changing any time soon, even though many people (and myself) think that certain other factors like social signals are becoming important.
A good link will send you nice link juice and help to boost your rankings so that you'll get more traffic and hopefully more conversions. A great link will do the same thing but it will send you traffic on its own.
Some links probably do absolutely nothing positive. You can get a link on a high-profile site and no one will ever click on it. You can receive referring traffic from a footer link on the crappiest site you've ever seen. You can get a rankings boost from both of those links. It's like magic.
Then there's the concept of authority. Links from other sites will lend credibility and authority to your site, ideally, through using you as an example. When a site links to you, the anchor text is viewed as an indicator of what your site is about.
Like the rest of this, that is no longer a perfect system. Theoretically, the keywords that a site links to you with should boost your authority for that topic.
If CNN linked to your site with an anchor of "great place to buy a computer" then your site would probably be viewed as an actual great place to buy a computer, and you'd probably rank higher for that phrase than if you'd gotten that link from your mom's local birdwatching site. However, the birdwatching site would still help you rank for a great place to buy a computer, but since it's most likely not as authoritative as CNN, to actually get a noticeable rankings boost, you'd need to get that link and more of the same for it to make a difference.
CNN has authority signals, which engines can take into account: people link to it, they reference it on Twitter and Facebook, they comment on stories, they comment on videos, the traffic is probably truly amazing, and the brand itself is one that most people recognize. One link from a site like that is much, much more powerful than more links from sites that have no social traction or online footprint.
Here is What I Truly Believe
The importance of links may lessen a bit, but it won't go away completely. The web was built on links. You can rank well without them of course (think breaking news stories or blog posts that get loads of attention on the first few days), but depending upon what shows up in a search engine's results is just as bad an idea as depending upon any one route into your site.