Ctrl/Cmd + S (Save) = incremental save of open file, no overwrite, change info is not deleted and can be accessed until next Save As or the file is closed. You continue to edit the same file.
Ctrl/Cmd + Shift + S (Save As) = create new file. If you use the same name existing file is overwritten and change info is lost. If you use a new name the old file is closed without saving any changes and any change data becomes inaccessible. You are now editing the newly created file.
Ctrl/Cmd + Alt/Opt + S (Save a Copy) = make a copy of the current state of the file in a location of your choice. This file is not opened automatically. You continue edit the same (old) file.