Linux is a true multi-tasking operating system. If you have an application that requires several processes to co-operate concurrently then Linux can arrange to have them run concurrently and take care of all the scheduling and administration of the processes on your behalf. In fact, you have already seen that this is so. Every time you use the shell to run a pipeline such as:
$ ls | wc -w
it involves multiple commands (ls and wc in this example) running concurrently and with a communication channel between them so that they can co-operate to perform a single task. When the shell arranges to run concurrent processes on your behalf it only uses ordinary user facilities which you are also free to use in your own programs. It is these facilities which form the subject of this tutorial.