WHAT IS A SYSTEM CALL

System call is a request made by any arbitrary program to the operating system for performing tasks -- picked from a predefined set -- which the said program does not have required permissions to execute in its own flow of execution. Most operations interacting with the system require permissions not available to a user level process, i.e. any I/O performed with any arbitrary device present on the system or any form of communication with other processes requires the use of system calls.
The fact that improper use of the system can easily cause a system crash necessitates some level of control. The design of the microprocessor architecture on practically all modern systems (except some embedded systems) offers a series of privilege levels -- the (low) privilege level in which normal applications execute limits the address space of the program so that it cannot access or modify other running applications nor the operating system itself. It also prevents the application from using any system devices (e.g. the frame buffer or network devices). But obviously any normal application needs these abilities; thus it can call the operating system. The OS executes at the highest level of privilege and allows the applications to request services via system calls, which are often implemented through interrupts. If allowed, the system enters a higher privilege level, executes a specific set of instructions which the interrupting program has no direct control over, then returns control to the former flow of execution. This concept also serves as a way to implement security.
With the development of separate operating modes with varying levels of privilege, a mechanism was needed for transferring control safely from lesser privileged modes to higher privileged modes. Less privileged code could not simply transfer control to more privileged code at any arbitrary point and with any arbitrary processor state. To allow it to do so would allow it to break security. For instance, the less privileged code could cause the higher privileged code to execute in the wrong order, or provide it with a bad stack.