Answer:
The difference between the "Save" and "Save as" buttons are as follows.
To save a document for the first time you need to select "Save as," because it has not been saved before, and so the computer doesnt know in which directory, folder, location you want to save the file on your hard drive.
The "Save" button is for if the document has been saved before, you have specified a save location. The computer will know where to save it. So it will simply re-save the document to that location.