Shell (the user interface, CLI or GUI), kernel (bridges hardware and software, manages resources), and hardware (the physical electronics).

1. Shell - The user interface that allows users to request specific tasks from the computer. Requests can be made through CLI or GUI interfaces.

2. Kernel - Communicates between the hardware and software of a computer and manages how hardware resources are used to meet software requirements.

3. Hardware - The physical part of a computer including underlying electronics.

Go deeper:

• Operating system — kernel, shell and components (Wikipedia) — how the kernel mediates between hardware and software, with the shell as the user-facing layer.
• Cisco IOS (Wikipedia) — the network-device OS that puts these same three layers behind a CLI.

Because a GUI can fail, crash, or hide options, while the CLI is lightweight, scriptable, consistent, and works even over slow links — so it is the reliable way to manage network devices.

A GUI is friendlier and needs less knowledge of the underlying command structure, but it depends on more resources and can fail, crash, or simply not expose everything a device can do. The CLI, by contrast:

• Is lightweight and runs even on low-resource devices
• Works reliably over slow or unstable remote links
• Is scriptable and gives consistent, repeatable results
• Exposes the full command set with direct access to the configuration

For these reasons, network devices are typically managed through the CLI.

Go deeper:

• Jeremy's IT Lab — Intro to the CLI (CCNA Day 4) — a hands-on tour of the Cisco IOS command line and why network engineers live in it.
• Command-line interface (Wikipedia) — why text commands are scriptable, repeatable and lightweight compared to a GUI.