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On the Mars Curiosity rover, there are black and white circular patterns or marks spaced around the rover, as seen here marked in lime green:
What are these?
If I had to guess, I'd wager they're points of reference, like to determine alignment of the rover and its parts, and/or the relative size/position/distance/focus of things, whether on-rover or in the surrounding environment. They also look very similar to patterns found on "focus cards" used for cameras.
They are called fiducial markers, or fiducials for short, and serve as calibration targets for onboard cameras and at the same time to allow the rover to measure itself and distance on photographs and other measuring devices, sensors, and mechanisms on the Curiosity rover by referencing two or more such marks as reference points, since their position and distance between them is known. The pattern itself can be also used to check for sharpness, focus and contrast.
If you look carefully on the self-portrait photograph of the Curiosity rover, you will also notice various color markers at the back, left from the RTG (Radioisotope thermoelectric generator) that serve as color calibration targets, one such set of many installed on the rover. As an interesting fact, other such targets also include a Lincoln cent.
