Explanation:
A system call is a fundamental mechanism that provides an interface between user applications and the operating system. It allows user-level processes to request services from the kernel, such as process control, file management, and communication. Unlike regular function calls, system calls switch the process execution from user mode to kernel mode, granting the process access to privileged operations.

System calls are typically categorized into five major types: process control (e.g., fork(), exec()), file management (e.g., open(), close(), read(), write()), device management (e.g., ioctl(), read()), information maintenance (e.g., getpid(), time()), and communication (e.g., pipe(), socket()). These categories enable efficient execution of tasks requiring system-level operations.

Modern operating systems optimize system calls to minimize context-switching overheads. For example, UNIX-based systems use a system call interface that provides secure access to resources without compromising system stability.

Formal Definition:
A system call is a mechanism through which a user process requests a service from the operating system’s kernel, enabling secure access to hardware resources and system-level functions.

Additional Information:
The implementation of system calls is hardware-dependent. In most systems, an interrupt mechanism is used to switch between user mode and kernel mode during a system call. A user application invokes a system call by writing its parameters in registers or using a stack structure. The operating system validates these requests and ensures secure execution.

System calls are vital for achieving multitasking, multiuser environments, and efficient resource utilization. Common examples include handling keyboard input/output, file access, and inter-process communication. Developers must carefully handle system calls to prevent security vulnerabilities and system instability.