Kernel space and user space

System calls are used when a user space program wants to use some data or some service provided by the Linux kernel. In current Linux kernel (2.6.23) there exist 324 system calls. All system calls provide a function which is useful for many application programs (such as file operations, network operations or process related operations). There is no point in adding a very specific system call which can be used only by a specific program. http://www.kernel.org/doc/man-pages/online/pages/man2/mknod.2.html

A conventional computer operating system usually segregates virtual memory into kernel space and user space. Kernel space is strictly reserved for running the kernel, kernel extensions, and most device drivers. In contrast, user space is the memory area where all user mode applications work and this memory can be swapped out when necessary.

Similarly, the term userland refers to all application software that runs in user space.^[1] Userland usually refers to the various programs and libraries that the operating system uses to interact with the kernel: software that performs input/output, manipulates file system objects, etc.
Each user space process normally runs in its own virtual memory space, and, unless explicitly requested, cannot access the memory of other processes. This is the basis for memory protection in today's mainstream operating systems, and a building block for privilege separation. Depending on the privileges, processes can request the kernel to map part of another process's memory space to its own, as is the case for debuggers. Programs can also request shared memory regions with other processes.
Kernel space and user space is the separation of the privileged operating system functions and the restricted user applications. The separation is necessary to prevent user applications from ransacking your computer. It would be a bad thing if any old user program could start writing random data to your hard drive or read memory from another user program's memory space.
User space programs cannot access system resources directly so access is handled on the program's behalf by the operating system kernel. The user space programs typically make such requests of the operating system through system calls.