Destructive (cannot undo once you save the file) (Pre Photoshop 3.0-4.0)
- select, copy and paste
- select and apply filters or painting directly into the image
Non-destructive (undo-able and infinitely redo-able) (Since Photoshop 4.0 on)
- use layers
- select and use layer-masks (instead of cutting an image up)
- select and use adjustment layers (instead of directly changing the pixels)
- smart objects (somewhat higher abstraction - will make sense once you know adjustment layers well)
If you accept destructive editing, photoshop is a very simple tool. If you can't do non-destructive editing, you probably won't get a job.
For the sake of this class, you must do non-destructive editing.