General-purpose input/output (GPIO)
A general-purpose input/output (GPIO) is an uncommitted digital signal pin on an integrated circuit or electronic circuit board which may be used as an input or output, or both, and is controllable by the user at runtime.
Video
Watch on Youtube here: https://www.youtube.com/watch?v=TSDS3op91TE
Internal Architecture
All STM32 MCUs have a high number of GPIOs. In fact, almost all pins on a MCU package, which is not dedicated a power pin are GPIOs:
The GPIOs are divided into a number of ports (the actual number depends on the pins available on any particular package) labelled with letters and starting with "PortA".
Each of these ports control up to 16 individual GPIOs each controlled individually through 4 32-bit control registers. The electrical architecture of each GPIO looks like this:
