Not all links are created equal Of course, not all links are equal. Some sites are more trusted than others.
So if they are more trusted, then links from those sites are worth more.
Likewise, some sites are more relevant than others to specifi terms.
The more relevant a site, the more value is transferred by the link.
Well known and established news sites, government sites (.gov) and university domains (.ac) are examples of sites from which links can carry more weighting.
SEOMoz.org, among others, provides useful tools which help you to determine the value of links from particular sites.