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The outermost level of the system is the one with which most users interact. This is the applications level. Applications are complex programs designed to automate some business task (such as word processing, spreadsheet calculations or database services).
Applications do not exist in a vacuum. Although a user may never see any other part of the system, the spreadsheet or word processor needs a way of storing files, producing a display on the user's terminal, and printing files out (to name just a few of its requirements). For example, the email application uses the lp(1) print service to print messages; this saves it from needing a print program of its own. Most applications behave like this, using lower level tools to carry out tasks which would otherwise have to be duplicated.
Applications also use system calls provided by the kernel. The kernel (the core of the UNIX system) provides a uniform interface to all the facilities of the system; this is described in more detail in ``The UNIX system kernel''.