An embedded system is a computer built into a larger product to carry out a dedicated function, rather than a general-purpose machine you load arbitrary programs onto.1
Overview
Most embedded systems are built around a microcontroller or system-on-chip running fixed firmware. They are everywhere — in appliances, cars, medical devices, industrial controllers, and radios — usually invisible to the user. Many have hard real-time constraints, met with interrupts and a real-time operating system, while simpler ones run bare metal. They read the physical world through sensors and act on it through actuators.
Where it fits
The defining trade-off is specialization: an embedded system does one job extremely well, cheaply, and reliably, at the cost of flexibility. When a device also needs network connectivity it becomes part of the Internet of Things. In radio terms, the small transmitters that fill the airwaves GopherTrunk listens to are embedded systems — far too small to run GopherTrunk itself, which lives on a general-purpose computer with an SDR front end.
Sources
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Embedded system — Wikipedia, on dedicated-function computers. ↩