Sequence of events:

Thermostat (1) calls for heat and checks all safety devices (2). Ignitor (3) (or safety pilot) is energized and gas valve (4) opens. Burner (5) lights. Heat accumulates in plenum (6), energizes blower control (7). Blower (8) drawing return air (9) over air filter (10) gains heat from passing over heat exchanger (11) and is distributed thru supply system (12).

When thermostat (1) is satisfied, the sequence is reversed. Gas valve (4) closes; burner (5) shuts off. Air gradually cools and blower control (7) shuts off blower(8).