A laptop or a smartphone is a general-purpose computer, built to run essentially whatever software a user chooses to install on it, from a web browser to a spreadsheet to a game. Embedded systems, built around small, purpose-specific chips called microcontrollers, work on a fundamentally different premise: a microcontroller is designed and manufactured to run exactly one fixed program for its entire operational life, controlling one specific device — a microwave's timer and heating cycle, a car's anti-lock braking system, a washing machine's wash cycle — and it's built into far more everyday objects than most people realise, since it's rarely visible or advertised the way a general-purpose computer is.
Fixed purpose lets a microcontroller be dramatically smaller and cheaper
Because a microcontroller only ever needs to run one specific, known program rather than an unpredictable range of arbitrary future software, it can be built with dramatically less processing power, memory and general-purpose flexibility than a laptop or smartphone's processor, while still reliably handling its one specific job perfectly well. This deliberate specialisation is precisely what makes microcontrollers cheap and small enough to embed economically into an enormous range of everyday products — a general-purpose computer's much greater processing power and flexibility would be complete overkill, and needlessly expensive, for a task as narrow and well-defined as running a microwave's timer.
The software is fixed at manufacture, not something the end user changes
A microcontroller's software, generally called firmware, is typically written once by the device's manufacturer and loaded onto the chip during production, with most consumer devices never expecting an ordinary end user to modify or replace that firmware themselves the way a general-purpose computer's owner routinely installs new software. Some more sophisticated modern embedded devices do support occasional firmware updates, generally issued directly by the manufacturer to fix bugs or add specific new features, but even that update process is still fundamentally different from a general-purpose computer's model of a user freely installing arbitrary new software — the microcontroller is still ultimately expected to run one manufacturer-defined program for the device's specific purpose, not an open-ended, user-chosen range of arbitrary applications the way a general-purpose computer is.
What we're still unsure about
The basic distinction between general-purpose computing and fixed-purpose embedded microcontrollers, and the cost and size advantages that specialisation provides, are well-established, uncontroversial computer engineering. What's more genuinely evolving, and a live area of both engineering practice and security research, is how much more general-purpose capability, including internet connectivity and remote firmware updates, should actually be built into embedded devices that were traditionally simple and fixed-purpose — adding that kind of connectivity and updatability brings real practical benefits, but it also introduces genuinely new security risks that a traditional, fully fixed, disconnected microcontroller never had to contend with, and engineers and security researchers continue actively working out where the right balance actually lies for different categories of embedded device.
This sits inside Embedded Systems & Microcontrollers, one of eight topics in Electrical Engineering, one of four domains in Engineering, one of seventeen subjects the app can quiz you on.