A unique link to a specific file on a file sharing network or system. It does not link to a specific location like hyperlinks do, instead merely containing the near-unique hash of the file that should be downloaded. Most file sharing programs generate the hash of the file on startup using standard MD4 or MD5 algorithms.
The user's file sharing client then tries to first locate the file by searching for its hash (unless the location is deliberately hidden, such as in Freenet) and then download it. This system is only slightly different from BitTorrent, where the process requires only the .torrent file and the very simple program install.
Example link for eDonkey2000 (modified to not work):
More discussion of the differences between different systems' hash-links is available on a Bitzi help page.