Every link, coming in from other sites, increases Your site's rating. This post is not how and where to get inbound links.
What is quality link?
- Your page is linked via anchor text, not just url
- Page and/or domain, what is linking to You, has PR>3
- Page, linking to You, has same keywords that Your page, relevancy
In general, more backlinks You got, better.
Google PageRank is complex mathematical model.
Image from Wikipedia
Shortly, when high-PR sites link to Your page, Your pages PR will increase. Having lot of links from PR=0 pages, doesn't increase it. Having links from black-listed sites may hurt Your page's rank.
Search engines value old links, but also link popularity trend. So, If Your page is getting more and more links, it should be quality site.