Link popularity as a search engine ranking factor is based on the notion that a link from one site to another is an editorial vote for the target site by the linker. Therefore, the idea goes, the more links from trusted websites a site or specific page has the better quality is can be assumed to be. It didn’t long after the introduction of link pop as a ranking factor for SEOs to begin gaming the search engines, by buying links, exchanging reciprocal links or setting up link networks. The key then for the search engines to combat link spam is to formulate intelligent algorithms to determine if a link is indeed an editorial vote. One of the easiest ways to do this is via block level page analysis (figuring out where the link is in the HTML on the page). Purchased and exchanged links are often found away from main page content, in sidebars or footers, so it can be assumed with a reasonable level of accuracy that links embedded in content blocks are not commercial or navigational, but real votes.
This is a key issue for modern link development, and nowhere is it more applicable or obvious that in blogs. Link ads and blogrolls (often large collection of run of site reciprocal links) appear in the sidebar while links to pages being discussed or recommended are found in the body of the page in posts.
Aside from the moral and ethical issues surrounding un-disclosed sponsored posts, these “pay for posts” models represent a great way to buy links nearly indistinguishable from purely editorial links.