A cell contains several kinds of machine, including, of course, the clients. A client is any machine which can see a cell's Coda file system.
In addition to clients, there is another kind of machine which we may call a file and data server, since Coda deals with a lot of data besides just files. One of the file and data servers plays a special role in a cell: the System Control Machine, or SCM. It is the master server, and needs to be installed first. And if you plan to backup your Coda system, you will need a backup machine.
A single machine may perform all four functions, so that you can have a cell with only one machine in it. Servers do not need to be clients in a Coda cell, nor do backup machines. But you will need more than one file and data server to take advantage of Coda's best features.
The organisation of a Coda cell has some implications for security: you need to protect the integrity of your file and data servers. In general, a hacker who breaks into a Coda client cannot compromise the security of the servers. Of course, if he lurks undetected, with root access, the blackguard may learn something compromising from another user of the cell.