It is basically the link to a site from outside of your site. It sends visitors away from your website.
Different sites links present in your site means different sites link pointing from your site is a outbound linking.
Outbound link is a link from your website going to another website. An inbound link is a link from another website linking to your website. An internal link is a link that links content within the domain of your website. Something you might find more interesting after learning the basics of linking is learning about link weight and link attributes. We did a few articles within the past few days about that very topic...link attributes... Promote DoFollow and Majority Set to NoFollow.
A link from one domain to another is said to be outbound from its source anchor and inbound to its target.
these are the links that a site is linking to.. the higher the number, the less value it has for your site (if you are between them) if you want to check this number, you could try this backlink checker, which is listing the OBL (outbound links) for sites that are linking to you!
Outbound link are responsible for linking your site to another domain. So any hyperlink that placed in your site using any anchor text direct another domain name is an Outbound link.
Outbound links can serve few purposes. By letting people post comments on your website with outbound links set to dofollow you spur people to comment more often. I use WordPress on all my websites by default WordPress sets comments to nofollow. The nofollow attribute which an attribute you can asign to url's was first introduced by search engines to help webmasters from being negatively affected by spam comments on their website. But with the advent of many spam plugins and only allowing comments you approve can allow legit people to benefit from commenting and commenting more often by promoting dofollow. This will generate more returning visitors. So the answer would be if search engines take into consideration spam follow outbound links and the possibility they could negatively affect your serp's then I would assume posting outbound links that are geared towards your niche and that are relevant will positively affect you. I put outbound links as low priority for SEO purposes. Just keep them relevant and you should not have a problem. See my article I created that is relevant to the topic about Promoting Do Follow http://nichewebsitestrategy.com/promote-dofollow