One post says a tuner is a human and another says a tuner is a device
The word "tuner" is used interchangeably but one is a tuning device and the other is a person who tunes the engine using a tuning device.
The tune (software & settings) that is in the engine controller (PCM or powertrain control module) can be modified by reflashing the PCM. To reflash the PCM a tuner (person) changes the software/settings and creates a file that can be loaded to a tuning device. The tuning device is connected to the PCM (via the car's OBD2 port) and the tuning device reflashes the PCM.