EXIF
The standard block of technical metadata a camera writes into a file, recording settings and equipment.
In depth
EXIF records the camera and lens, focal length, aperture, shutter speed, ISO, metering and focus mode, date and time, and often the shutter actuation count and GPS position. It is invaluable for learning from one's own work, and it can disclose more than intended — the location of a home, or the serial number of a camera.
Examples in practice
Reviewing the EXIF of a soft frame reveals it was shot at 1/30 s with a 200 mm lens, explaining the blur.