"Cropping an image" means some part of the image is removed in order for the image to fit a certain size. For example, removing the rightmost part of a rectangular image can result in a square.
"Resizing an image" usually means to stretch and squeeze it as needed so it fits a given size. Width and height are modified independently. If the ratio between them is not the same in the original size and in the final size, the image will appear distorted.
"Scaling an image" usually means resizing, but by the same amount horizontally and vertically. The ratio between the width and height is not modified, therefore the final image is not distorted.
The "aspect ratio" is the ratio between the width and the height. Changing the aspect ratio will perceptually stretch the image either horizontally or vertically.
Resizing might change the aspect ratio, and from the wording of your question I have assumed scaling meant not changing the aspect ratio.