Search algorithms also consider relationships between linked sites. By
analysing various things, the engines try to determine if the links are natural
links, or if they are manipulative, artifiial links created solely for ranking
purposes. Manipulated links are worth very little compared to natural links
and may lead to a drop in search engine rankings.
The search engine algorithm will also determine the relevancy of the referring website to the site being linked to.
The more relevant the sites are to each other, the better.
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.