The camera's shutter speed, the lens's aperture (also called f-stop), the film speed (a.k.a. ISO), and the scene's luminance together determine the amount of light that reaches the film or sensor (the exposure ). exposure value (EV) is a quantity that accounts for the shutter speed and the f-number. Once the sensitivity to light of the recording surface (either film or sensor) is set in numbers expressed in "ISOs" (ex: 200 ISO, 400 ISO), the light emitted by the scene photographed can be controlled through aperture and shutter-speed to match the film or sensor sensitivity to light. This will achieve a good exposure when all the details of the scene are legible on the photograph. Too much light let into the camera results in an overly pale image (or "over-exposure") while too little light will result in an overly dark image (or "under-exposure").
This is my favorite photo because the image is in focus and it has a fast shutter speed making the image crisp and clear.
This is my worst image because the image is completely out of focus and the exposure is too high making the image too bright, this accompanied with a slow shutter speed makes the image low quality.