The part of the Arduino that you’ll care the most about during your projects is
the general-purpose I/O and ADC pins. All of these pins can be individually
addressed via the programs you’ll write. All of them can serve as digital inputs
and outputs. The ADC pins can also act as analog inputs that can measure voltages
between 0 and 5V (usually from resistive sensors). Many of these pins are
also multiplexed to serve additional functions, which you will explore during
your projects. These special functions include various communication interfaces,
serial interfaces, pulse-width-modulated outputs, and external interrupts.