Raw Data
The minimally processed values read from the sensor, before demosaicing, white balance or tone rendering.
Measured in
ADU
In depth
A raw file is closer to a set of measurements than to a picture: each value is one photosite's electron count, and everything from colour to contrast is applied later as an interpretation. That is precisely its value — decisions such as white balance and tone curve remain adjustable with no loss, and the original measurement is never overwritten.
Examples in practice
Changing a raw file's white balance from 3200 K to 6500 K costs nothing in quality, where the same change to a JPEG would not.