The ed2k links are used by the eDonkey 2000 family of the P2P programs, such as eDonkey 2000 itself, eMule, Hybrid, MLDonkey and others. These links are primarily used to provide a unique identifier for file without refering to a specific location, like in ftp and http links. This is done by using the hash function on the file to calculate its checksum. The file is spread into 9.28Mb chunks and hash is calculated for every one of them. The resulting hash table is hashed once again and the final value is used as a part of the ed2k link.
The typical ed2k link also includes the filename and the filesize. An example (a link to the 15Mb Purist Edit trailer) is provided below:
The ed2k links can also include a reference to the IP address and port of the specific client that shares a file. This is done by adding a list of sources after the main part of the link.
The "file" part indicates that this is a file link, as opposed to server link. A server links is a reference to the eDonkey 2000 server. Clicking on such links typically adds it to the server list.
After installing on of the eDonkey 2000 clients the ed2k:// protocol is registered in Windows (and perhaps, other OSes), so that clicking on it is browser automatically starts the download.