In addition to rankings by content, many search engines rank pages based on inbound links. Google's description of their PageRank system, for instance, notes that Google interprets a link from page A to page B as a vote, by page A, for page B. Knowledge of this form of search engine rankings has fueled a portion of the SEO industry commonly termed linkspam, where a company attempts to place as many inbound links as possible to their site regardless of the context of the originating site.
Increasingly, inbound links are being weighed against link popularity and originating context. This transition is reducing the notion of one link, one vote in SEO, a trend proponents hope will help curb linkspam as a whole.