Eurotherm 821 is a controller/programmer with a 9-segment error meter, 4-digit indicator, and a 15 x 54 dot matrix display. It features a multifunction key to access various functions, including run, reset, hold, and enter/yes. The device also has up/no/manual, cancel/shift, down/automatic, and hold/reset keys. It can be used in various applications, including temperature control, process control, and automation.