The Kernel
The kernel is the part of the OS that stays in memory and runs in a privileged mode with full access to the hardware. It mediates every request a program makes for the processor, memory, or devices.
Process & CPU scheduling
A process is a running program. The scheduler decides which process gets the processor next and for how long, switching between them quickly enough that they appear to run at the same time.
Memory management
The OS assigns each process its own region of memory and uses virtual memory to give programs the illusion of more space than is physically installed, paging data to disk when needed.
File systems
File systems impose structure on raw storage, tracking where each file's data lives and enforcing permissions on who can read or change it.
Device drivers & I/O
Drivers translate the OS's generic requests into the specific signals a given piece of hardware expects, so the same program can print to many different printers.