Hardware and software

Hardware is the physical parts of a computer. Software is the programs that run on it.
System software runs the computer, such as the operating system. Application software does jobs for the user, such as word processors, browsers and games.
Input devices include keyboards, mice, microphones, scanners, touchscreens and sensors. Output devices include monitors, printers, speakers and motors.
An embedded system is a computer built into another device, such as a washing machine, microwave or car.

The CPU

The CPU (central processing unit) processes data and carries out instructions, repeating the fetch-decode-execute cycle.
Clock speed (in GHz) is how many cycles it carries out each second. More cores let it do more tasks at once. Cache is a small amount of very fast memory inside the CPU.

RAM holds the programs and data in use. It is volatile: it loses its contents when the power is turned off.
ROM is non-volatile and holds the start-up (boot) instructions.
Secondary storage keeps files when the power is off: hard disk drives (HDD: cheap and large, with moving parts), solid-state drives (SSD: fast, with no moving parts), optical discs (CD, DVD, Blu-ray), flash memory (USB sticks, SD cards) and cloud storage (remote servers reached over the internet).

The operating system manages the processor and memory, provides a user interface, manages files, controls devices through drivers, and keeps the computer secure with user accounts and passwords.

Hardware: The physical parts of a computer.
Software: The programs that run on a computer.
Operating system: System software that manages the hardware and runs other programs.
Application software: Programs that do jobs for the user, such as a word processor.
CPU: The central processing unit, which processes data and carries out instructions.
Clock speed: How many cycles the CPU carries out each second, measured in GHz.
RAM: Volatile memory that holds the programs and data in use.
ROM: Non-volatile memory that holds the start-up instructions.
Volatile: Loses its data when the power is turned off.
Secondary storage: Non-volatile storage that keeps files permanently.
SSD: A solid-state drive: fast storage with no moving parts.
Embedded system: A computer built into another device.