A kernel is a computer program at the core of a computer's operating system that always has complete control over everything in the system. The kernel is also responsible for preventing and mitigating conflicts between different processes. It is the portion of the operating system code that is always resident in memory and facilitates interactions between hardware and software components. A full kernel controls all hardware resources (e.g. I/O, memory, cryptography) via device drivers, arbitrates conflicts between processes concerning such resources, and optimizes the use of common resources, such as CPU, cache, file systems, and network sockets. On most systems, the kernel is one of the first programs loaded on startup (after the bootloader). It handles the rest of startup as well as memory, peripherals, and input/output (I/O) requests from software, translating them into data-processing instructions for the central processing unit.

The critical code of the kernel is usually loaded into a separate area of memory, which is protected from access by application software or other less critical parts of the operating system. The kernel performs its tasks, such as running processes, managing hardware devices such as the hard disk, and handling interrupts, in this protected kernel space. In contrast, application programs such as browsers, word processors, or audio or video players use a separate area of memory, user space. This prevents user data and kernel data from interfering with each other and causing instability and slowness, as well as preventing malfunctioning applications from affecting other applications or crashing the entire operating system. Even in systems where the kernel is included in application address spaces, memory protection is used to prevent unauthorized applications from modifying the kernel.

The kernel's interface is a low-level abstraction layer. When a process requests a service from the kernel, it must invoke a system call, usually through a wrapper function.

There are different kernel architecture designs. Monolithic kernels run entirely in a single address space with the CPU executing in supervisor mode, mainly for speed. Microkernels run most but not all of their services in user space, like user processes do, mainly for resilience and modularity. MINIX 3 is a notable example of microkernel design. Some kernels, such as the Linux kernel, are both monolithic and modular, since they can insert and remove loadable kernel modules at runtime.

This central component of a computer system is responsible for executing programs. The kernel takes responsibility for deciding at any time which of the many running programs should be allocated to the processor or processors.

Random-access memory (RAM) is used to store both program instructions and data. Typically, both need to be present in memory for a program to execute. Often, multiple programs will want memory access, frequently demanding more memory than the computer has available. The kernel is responsible for deciding which memory each process can use, and determining what to do when insufficient memory is available.

I/O devices include, but are not limited to, peripherals such as keyboards, mice, disk drives, printers, USB devices, network adapters, and display devices. The kernel provides convenient methods for applications to use these devices which are typically abstracted by the kernel so that applications do not need to know their implementation details.

Key aspects necessary in resource management are defining the execution domain (address space) and the protection mechanism used to mediate access to the resources within a domain. Kernels also provide methods for synchronization and inter-process communication (IPC). These implementations may be located within the kernel itself or the kernel can also rely on other processes it is running. Although the kernel must provide IPC in order to provide access to the facilities provided by each other, kernels must also provide running programs with a method to make requests to access these facilities. The kernel is also responsible for context switching between processes or threads.