In Internet marketing there was once something called link farming where a website has a list of links that are all hyper linked to one website. They use this to gain link popularity and higher ranks on a search engine. Link farms can be created by hand or sometimes may be created by automated programs that hackers have created.
A search engine finds link farming a form of spam and marks the site as a spamming site and it does not become rated to be in the top search results. Link exchanging systems are originally designed to allow actual person based websites to exchange links with other actual websites and are not considered a form of spamming, spamdexing, or link farming.
Link farming was a large part of finding useless websites when a person would enter keywords into a search engine because when they would spam links it would show their popularity as being high so they were put on the top of the search list. Link farm exchanges were dealt with informally before but several service companies were found to provide automated forms and member updates to member websites within the link exchange.
New link finding software emerged to find potential link partner sites. They sent the users to webpage based emails offering to exchange links with directory link pages for websites in hopes of building their link popularity and page rank on the search engines page. This is detected by the new software more recently so it happens less often now.