Perhaps you've never thought about files as an IPC mechanism before, but they shoulder the lion's share of interprocess communication--far more than all other means combined. When one process deposits its precious data in a file and another process later retrieves that data, those processes have communicated. Files offer something unique among all forms of IPC covered here: like a papyrus scroll unearthed after millennia buried in the desert, a file can be unearthed and read long after its writer's personal end. Factoring in persistence with comparative ease of use, it's no wonder that files remain popular.
Using files to transmit information from the dead past to some unknown future poses few surprises. You write the file to some permanent medium like a disk, and that's about it. (You might tell a web server where to find it, if it contains HTML.) The interesting challenge is when all parties are still alive and trying to communicate with one another. Without some agreement about whose turn it is to have their say, reliable communication is impossible; agreement may be achieved through file locking, which is covered in the next section. In the section after that, we discuss the special relationship that exists between a parent process and its children, which allows related parties to exchange information through inherited access to the same files.
Files certainly have their limitations when it comes to things like remote access, synchronization, reliability, and session ...