Coda is one of several relatives of AFS, which resemble each other in many respects, and which differ from other shared files systems.
AFS itself has been a commercial product since about 1987. It still exists, and it got a new lease on life when IBM bought Transarc. AFS is very well documented, and the documents are all available free on line; it's also discussed in a number of books about file systems. If you're interested in Coda, some of this information may be relevant. I'll say something about Coda's own documentation later.
The Arla project at the Royal Technical Hochschule (KTH) in Stockholm is an attempt to create a free clone of AFS that interoperates with it. Arla has concentrated on duplicating the AFS client software and the user interface, though some work is now being done on Arla servers. This project is being encouraged by IBM and Transarc, who have donated some code to it, and some of the early AFS developers are helping out in small ways, at least.
Coda is a research project that began at CMU about 1987. It was designed as an AFS-like file system which supports mobile computing, and which is more robust than AFS when it faces network problems and server failures. A paper which appeared in 1987 says that the design of Coda is complete, but that no code has yet been written. The file system has been developed at CMU since then by the systems group of M. Satyanarayanan.
DFS is a file system that is based on AFS and was originally intended to supersede it. Several major computer manufacturers, including IBM, DEC, and HP, put together a 'Distributed Computing Environment' which they hoped would become a sort of industry standard; the 'Common Desktop Environment' CDE is perhaps the most successful piece of DCE. DFS itself has never really caught on, partially because of some early difficulties in getting it working, and partially, in my view, because of the excessive licence fees.
I have had some experience with all four of these systems, though I have seen relatively little of DFS, because we in Manchester decided not to buy it.
These file systems are quite different from NFS or the other systems I have named. In some ways, the structure of data in AFS and DFS is managed globally, so that all AFS or DFS users in the world see the same file system. If I make a change to my AFS cell, it is visible immediately not only to all the machines in my cell, but to everyone else as well. Coda lacks the global aspect; so far as I can see, I can't make two Coda cells visible on one client (at least not without running two independent Coda client processes for two different mount points). But Coda management is cell-based, like that of AFS and DFS. In other words, if I mount something in the file system, it immediately appears everywhere in the cell.