I'm thinking it's light sensitive receptors, but then it could possibly be either pixels or film as well. I'm not really sure. So, if anyone is completely certain of what is etched into the surface of a digital camera sensor, please let me know.
At the most basic level, digital camera sensors are arrays of photodiodes, semiconductor devices that generate electric current when exposed to light. The means by which photodiodes create electron-hole pairs when exposed to sufficiently energetic photons is called the photoelectric effect, described by Albert Einstein in a 1905 paper, and for which he won the 1921 Nobel Prize ("for his discovery of the law of the photoelectric effect").
There are different types of digital camera sensors (CCD, CMOS, etc.), but at a fundamental level, their light collection cells — pixels, "sensels" (sensor pixel), photosite, whatever you want to call them — operate on the photoelectric effect.
Regarding how the photoelectric effect works, or how semiconductors are etched onto silicon, that's beyond the scope of Photo.SE. I suggest some of the following questions (amongst others) at sister Stack Exchange sites:
- How are integrated circuits fabricated? (Electrical Engineering Stack Exchange)
- Understanding the principle of photodiode signal generation (Physics Stack Exchange)