In a nutshell
Strictly, Linux is just a kernel — the core that Linus Torvalds released in 1991 — but bundled with the shell, the GNU tools, and the C library it makes a complete, open-source, Unix-like operating system. It is monolithic: the scheduler, memory manager, file system, and drivers all run together in one privileged space. A typed command flows app → shell → library → system call → kernel → hardware.