If the image has not previously been saved, then "Save" does the same thing as "Save As".
If you quit your image without saving it, a message will ask you if you actually want to do so, provided that the "Confim closing of unsaved images" option is checked in the Preferences/Environment dialog.
The "Save" command, activated from an image menu as
-> ,or from the keyboard using the shortcut Ctrl-S, saves the image to a file. If the image has been saved previously, it will automatically be saved to the same location, using the same file name, file type, and options. To save it differently in any respect, use either Save As or Save A Copy.