Generally, the technical image quality of a digital image (before we get to the post-processing stage) is the product of the characteristics of the lens and the recording medium, with an important dash of good technique from the photographer.
Lenses are covered under What characteristics make a good lens good?, but what about the other important component, the sensor? DxOMark famously publishes some measurements, but what's really important? Obviously megapixels are part of it, but what else matters? Are there non-lens camera features other than the sensor which can have a positive or negative effect on image quality?
Most importantly, what do these things look like in actual real-world images, and how can I recognize them by looking it photographs rather than just review charts and graphs?