Fit within scales the image so it fits inside the box you type, keeping its shape. A 4000×3000 photo with a 1920×1920 box becomes 1920×1440. Nothing is cropped.
Exact size (crop) produces exactly the pixels you type. If the shape differs from the original, the tool crops around the position you choose by dragging the preview. This is what forms, thumbnails and profile pictures need.
Keep original pixels changes nothing about the size and only re-encodes, which is useful for converting formats or lowering the file size with the quality slider.