Quality Link

Search engines count links votes of trust. Quality links count more than low quality links.
There are a variety of ways to define what a quality link is, but the following are characteristics of a high quality link:

1. Trusted Source: If a link is from a page or website which seems like it is trustworthy then it is more likely to count more than a link from an obscure, rarely used, and rarely cited website. See TrustRank for one example of a way to find highly trusted websites.
2. Hard to Get: The harder a link is to acquire the more likely a search engine will be to want to trust it and the more work a competitor will need to do to try to gain that link.
3. Aged: Some search engines may trust links from older resources or links that have existed for a length of time more than they trust brand new links or links from newer resources.
4. Co-citation: Pages that link at competing sites which also link to your site make it easy for search engines to understand what community your website belongs to. See Hilltop for an example of an algorithm which looks for co-citation from expert sources.
5. Related: Links from related pages or related websites may count more than links from unrelated sites.
6. In Content: Links which are in the content area of a page are typically going to be more likely to be editorial links than links that are not included within the editorial portion of a page.
7. While appropriate anchor text may also help you rank even better than a link which lacks appropriate anchor text, it is worth noting that for competitive queries Google is more likely to place weight on a high quality link where the anchor text does not match than trusting low quality links where the anchor text matches.