ed2k links are essential part of the success of eDonkey2000 network that has grown to be one of the largest P2P networks in the world.
ed2k links are actually just hashed checksum values for files shared over the eDonkey2000 network, thus pointing to files in the P2P networks without actually defining their specific location (that means that the very basic ed2k link doesn't indicate who is sharing the file, but simply points to a file with that checksum value within the network, no matter who has it). Due its technical specification, ed2k links can also be "dead", meaning that the checksum value points to a valid file in the network, but no one in the network is (currently) sharing it, thus the file doesn't have any sources available.
Opening an ed2k link requires an eDonkey2000 -compatible P2P client. Such clients include eMule (open source), eMule Plus (open source) and eDonkey.
Note that ed2k links can, to the contrary of the above statement, also include the specific IP address and the port of the person who is sharing the file. Such ed2k links are rarity in the eDonkey2000 network, but as stated, they can be created.
Basic ed2k file link is built like this:
ed2k://|file|the-file-name|file-size-in-bytes|hash-string|/
Additional information for IP address and ports are added after the hash string. An example of this:
ed2k://|file|the-file-name|file-size-in-bytes|hash-string|/|sources,127.0.0.1:4662|/
In the above example 127.0.0.1 indicates the IP address of the sharing source and the 4662 indicates the port of the sharer's client.
Additionally, ed2k links can also be "server links", pointing to a server instead of a shared file. In this regard, it is important to understand that eDonkey2000 P2P network differs from many other modern P2P networks due the fact that it is not totally decentralized, but instead it is spread across independent servers that act as hubs to connect peers to each others. ed2k server links are formed like this:
ed2k://|server|127.0.0.1|4242|/
In above example, the 127.0.0.1 indicates the server's address and the 4242 indicates the server's port.
For more information about eDonkey/eMule themselves, we recommend visiting these links:
eMule tutorial
AfterDawn's P2P forum
