This article is definitely more technical than average, but it's an important concept for people really wanting to know how their image tones are recorded:
Tutorial: Understanding Gamma Correction & Digital Tones
It is also probably one of the most misunderstood. Regardless, understanding how gamma works can improve one's exposure technique, in addition to helping one make the most of image editing.
As usual, please let me know if you feel anything is unclear, if you notice any typos or just want to add something from your own experience.
PS: this was the tutorial that was recently referred to in this thread: Gamma: Overrated?