Its well known ( or rather should be known ) that the immense relevance of Google's ( and for that reason other popular search engines ) search is owning to the weight it gives to the links that are contained in a web page. It was counting on that that link spamming originated.

The technique was about putting as many links as possible online for the site that was to be promoted and more links meant more importance for the site and so the search engines were foxed. But that did not last for long and search algorithms were tweaked to black list sites that were following negative tactics to promote links.

Link spamming then saw the emergence of link farms - a wide net of websites that linked to one another. The technique was successful till search engines were tweaked to rank down such sites.

From there link spamming has moved to attacking blogs and wikis in a big way. Unlike web content in the early days, blogs and wikis are constantly updated and new content is added quite frequently. This provided an ample opportunity for large scale link creations.

Search engines have been promoting the use of the nofollow tag so that crawlers can be prevented from accounting the links that occur in blogs. But that is another story.

Here's an excerpt from an interview with a link spammer (Register)

Here's why. When Sam spams tons of blogs and sites with links to his sites - which are affiliates of bigger PPC sites - people see the links and, seeking some porn, pills or casino action, click through to his site, and from there to the parent site, which pays Sam for each person landing there. The PPC sites can see revenues of £100,000 to £200,000 per month, says Sam. He gets a slice of that - and he wants it to stay that way.

So that is about Link spamming and yes, if you really want to get ranked high on search engines focus most on the content and how you can make what you post make a difference to those who read it. Once that happens, the traffic will follow.