What is the difference between digital and analog I/O in PLCs?

Digital I/O handles discrete (on/off) signals: inputs from switches, proximity sensors, pushbuttons; outputs to solenoid valves, indicator lights, motor starters. Analog I/O handles continuous signals: inputs from transmitters (4-20 mA, 0-10 V, thermocouples, RTDs); outputs to control valves, variable speed drives. Digital modules are simpler (typically 8, 16, or 32 points); analog modules include A/D or D/A converters and are specified by resolution (12-bit, 16-bit), range, and accuracy.