The kernel is the central part of an operating system that manages input/output requests and translates them into instructions for the CPU and other components. It is responsible for memory management, allocating processes to the CPU, and handling input/output from devices. The basic structure of a kernel includes facilities for the CPU, computer memory, and input/output devices. Kernels can take different forms such as monolithic, micro, hybrid, nano, or exokernel depending on their modularity and how they expose hardware resources to other parts of the system.