UNIX is a multi-tasking operating system, which means that a number of programs can run at the same time. Those programs (called processes) can communicate with each other.
For example, a C program could be compiling as mail is being read or a file is being edited.
Processes that ``wake-up" occasionally, and/or regularly, are called daemons. Daemons are used to synchronise disks, send and receive mail, print documents, and so on.