Noise is the digital equivalent of grain in film. It’s random electrical signals captured by the photosites on the camera sensor, and usually this background noise level is so low compared to the brightness of the captured picture itself that you just don’t notice it.
Sensor size keeps coming up in any discussion about image quality. Megapixels are a factor too, but the size of the sensor is ultimately more important. Bigger sensors have bigger photosites which capture less noise and more dynamic range.
The aspect ratio of an image is the ratio of its width to its height. The larger the ratio, the ‘wider’ the image; the smaller the ratio, the ‘squarer’ the image.