Destructive editing does not necessarily cause harm to your picture or ruin it in anyway, as long as you know what you’re doing. Destructive editing is more or less improving your image by manipulating pixels of the digital photograph. When making changes to the photograph as a part of the editing process is considered “destructive,” making changes permanently to the pixels. When manipulating an image for example: when darkening an image or area of an image, the pixels within targeted boundaries will be altered to become darker. Whenever the image is saved. the subjected pixels will be written into the image permanently and can not be returned to their original state. Hence, the term “destructive editing” since the original state of the pixels are considered to have been destroyed. Destructive editing is the common way of editing digital photographs because for many years it was the only way to alter digital photographs. Now its possible to edit a digital photograph in Photoshop in a way that it is not destructive by taking advantage of layers with a little more experience in Photoshop, more information of destructive editing can be found here.
Non-destructive edits do not change the original data there are various examples of non- destructive editing. Lets say you have a photograph and cant choose between either a colored version or a black and white. There can be destructive editing and can be considered non destructive because of working on different layers, instead of working on the original layer. Nondestructive lets you export two copies of the original photograph, one in color and the other in black and white. There are various advantages of nondestructive editing like image quality, flexibility, and preservation of the original data. Preserving the unaltered state is for future edits, on the other hand if the original data is edited permanently then there is no return. There are techniques for editing by using tools that are extremely flexible. Retouching on a separate layer with clone stamp, healing brush, or spot healing brush tools are all tools that let you retouch non-destructively on a separate layer. After creating a cropping rectangle with crop tool, selecting hide from the options bar to preserve the cropped area, and will be able to restore the cropped out portion. When transforming with smart objects allows nondestructive scaling, rotating, and warping. More information of non destructive editing can be found here.