In all the documentations of X11 that I've found so far something like this is written
Communication between server and clients is done by exchanging packets over a channel. The connection is established by the client (how the client is started is not specified in the protocol). (from wikipedia)
I haven't been able to find what is this channel exactly? A network channel for example? Is it on a port? Is it a memory map? Any help is appreciated.
The phrasing of 'channel' is intentionally vague as it can be either over a local socket, a remote connection (such as SSH), a named pipe, or another method that allows client/server bidirectional communication. Which is to say, a 'channel' is simply a connection between two points that facilitates exchange of data.
When perform X11 forwarding over SSH, the channel is the SSH connection. See the SSH man page for example:
$ man ssh
X11 connections and arbitrary TCP/IP ports can also be forwarded over the secure channel.
or per the x.org documentation:
The communications channel between an X client and server is full-duplex: either side can send a message to the other at any time. This is canonically implemented over a TCP/IP socket interface, though other communications channels are often used, including Unix domain sockets, named pipes and shared memory. The channel must provide a reliable, ordered byte stream---the X protocol provides no mechanism for reordering or resending packets.
X11 support multiple forms of communication between client and server. These so called channels can be TCP sockets, UNIX sockets, and a bunch of other network mechanisms, such as DECnet, token ring etc. TCP and UNIX sockets are really the only ones used today.
The X server is a process that has access to the graphics hardware, keyboard, and mouse. Any application that produces graphics on the computer screen is called a client. Usually, a workstation has on X server running, and multiple X clients. The applications (clients) need to connect to the X-Server via a TCP socket (identified by IP address and port number) or via a UNIX socket (identified by a file name, e.g. /tmp/X0)
If both, server and clients, run on the same system they usually connect through the UNIX socket. However, one of great features of X11 is that server and clients do not have the reside on the same system, but rather connect through the network via TCP sockets. This allows us to run applications on different computers on the network, and bring their graphics output on a single screen. (A single application may also connect to multiple X server and distribute graphics content on multiple screens.)
