Aperture controls lens’ diaphragm and is indicated in f-stops. The lower f-stop is, the more light is coming through the lens. The aperture also controls DOF - the depth of field. The low aperture causes shallow zone in focus on the picture and blurry background. This is useful for portraits. The higher f stop is, the more picture is in focus. Depth of Field is a zone of acceptable sharpness within a picture.

Shutter speed indicates how fast a camera's curtain opens and closes and is measured in a fraction of seconds i.e. 1/60,1/2000. Shutter speed also controls the motion on the picture. The slower shutter speed is the more motion is captured. The faster shutter speed less motion is visible and the action (movement) is frozen on the picture. Below bird's wings are blurry because the shutter speed was too slow to freeze the bird in flight and the movement is visible. 

ISO controls the sensitivity of the camera’s sensor. The lower the ISO rating, the less sensitive the sensor is, and the smoother the picture because there is less digital noise. Pictures taken with High ISO 25000 or 50000, more noise is visible so if you crop the image the details won’t be smooth and the grain will be visible.