also known as: backlink
A link that is coming into a given webpage. Webloggers and linkloggers make links all the time, but the basic Web is made of unidirectional links, which means it doesn't always hand inbound links to you on a silver platter -- you generally have to dig them up.
Some pages explicitly display their inbound links; this helps visitors get a sense of the relationship of the page to the rest of the Web. Others do not, but this does not mean it is impossible to retrieve them: LinkEcosystemServices will let you find inbound links for pages (in Google or Yahoo, for instance, prefix the URL you want to look up with "link:"; the results are rarely comprehensive). A number of handy bookmarklets let you query them to look up backlinks when viewing webpages in your browser, simply by clicking a button on your links bar.
Apart from resorting to link ecosystem services, some widespread mechanisms to track inbound links are under the page owner's control. These include referer tracking and the TrackBack protocol.
Inbound links are interesting for several reasons. First, they enable DecentralizedAnnotation: They enable anyone to comment on any page from whatever site they can write on. I use the Bloglines and Technorati bookmarklets all the time to know what others have been saying about what I'm reading.
Also, inbound links also enable GroupForming: given a means of being aware of inbound links, URLs can become RendezVous, gathering spots for people who have something in common.
Finally, tallying inbound links for a page is one basisfor measuring how valuable/popular it is. See LinkRank.