Non-destructive editing (NDE) refers to editing techniques or features that allow the original contents to be preserved, while also allowing for visible changes to be made.
In practice, painting and retouching tools, filters, and adjustments make what are called destructive edits—certain pixels are altered. Further edits will call on the newly-changed data, as the original data is no longer available. Though many image editing programs have an Undo command, use of the command is often limited to one edit, with the next use either being not available or "undoing the undo". Unless a backup copy was made in advance, saving the image will overwrite the original image data forever.
Meanwhile, non-destructive editing keeps the editing data separate from the original image data. Changes can be made to the editing data rather than the original image, adding versatility to the process. (Changes can also be made to the original data, though these will be destructive—the editing data will further cause a visible non-destructive change.) Because the editing data is kept separate, more RAM is needed by the program.
NDE in Photoshop
Starting with version 3.0, a feature called Layers was added, and Photoshop became capable of non-destructive editing. Edits could be made to a new layer, while the original image remained on a lower layer. If a filter or adjustment was needed, it could be applied to a duplicate of the original image layer. The user only had to ensure that the layer selected for editing was not the original.
Layers also allowed for different compositions to be made in a single document. The edits in one layer could be made invisible, and a different layer made visible. An editing layer could itself be duplicated, and further edited to produce the new comp.
Version 3.0 also added vector capability in the form of Paths and the Pen tool. Because vector objects are not dependent on raster data, they can be edited independently. Other vector features would be later added, including the ability to Place into an image as a vector object.
In time, other features would be given their own NDE capability, making Photoshop among the premier image editors.