Outbound Links: An outbound link is a link that appears on one website to direct users to another, external website.
Some websites may post outbound links that have nothing to do with their websites (a tactic known as link hoarding). In most cases, however, using outbound links within the context of your content is a good way to let search engines understand what your website is about. Referencing other, more well-known sources can boost your site's credibility.
Webmasters who would like to know which other websites use outbound links to their website can check this on Google using the "link:" function. These results would be referred to as inbound links. Other webmasters may use this to find sites that have linked to them, and if yours is one of them, they may link back to you in turn if they have found your content useful.
Some sites mark outbound links, so users will know the difference between those and internal links. Wikipedia, for example, signifies an outbound link with a small arrow next to it. Not all websites use this practice, though. This does provide a good example, however, of using outside sources to boost a site's credibility.