Back-links have long been considered in Google's ranking algorithm as a mechanism for casting votes for a target website. The more back-links, the more votes, the more 'important' the target page and the higher it would rank in the SERP's. As always happens, people sought to exploit back-linking to their advantage by automating the generation of large amounts of back-links in an effort to manipulate the Google SERP's. And it worked fine for a time. But the benefit given to these back-links by Google is changing. Going into 2011 it was generally understood that Google utilized two related ranking factors for all web content. The first was 'Page Rank'. This was a popularity score or vote given to a page calculated on the number of sites back-linking to that page. The actual PR was calculated based on an evaluation of each incoming link performed within the Google software or 'algorithm'. The evaluation considered the PR of the referring page and how many outbound links were on that page. Factors such as the referring site 'authority' and the referring site theme, were also assumed to be passed along with these links.

The last two components of the back-link value are still important – referring site authority and theme or relevance. These are difficult to measure in ways where a numerical value can be assigned, so many people with a 'scientific' bent often dismiss their existence (if it can't be measured it doesn't exist). But 'site authority' and the value of the referring site's relevance do exist. In simple terms, a site builds authority as a whole entity, whereas individual pages in a site build individual Page Rank. Sites like Amazon.com, Youtube.com have high site authority. Individual pages within the site may not have a high PR, but when you receive a link from a PR0 page on a high authority website it carries with it a little more juice (importance and benefit) than a link from a PR0 page on a low authority website.
The sites' relevance to your theme is even more important. If your website is all about 'natural health', having a link from a site also about natural health will improve the juice flowing from that link.

So how does that apply to a link coming from an Article Directory back to your website?

Well, I have this answer and many more, along with a complete strategy for marketing your website using articles and article syndication - right here.