IQ or Image Quality encompasses all aspects of how a camera and lens capture a scene.
There are a great number of aspect which comprise image quality and they may be attributed to different parts of the camera or lens:
- Image noise
They apply differently to different cameras and some such as color-accuracy only apply to an image processed in-camera rather than a RAW file.
Image quality is a property of the camera and lens at a certain setting. Using a different ISO, shutter-speed, image-parameters (JPEG), aperture or focal-length can result in different image quality from the same camera and lens combination.
Just note that many people confuse the quality of an image with its success. A low quality image can be very successful and vice-versa. Success has much more to do with the photographer's skill, subject and moment than it does with the camera and lens.