OOOh OOOOh - I can answer that for you. Save as... is used when you want to change a file name - such as adding 'copy' or something to your existing file name, or move it to a different directory (folder).
Save simply saves you painting file with the same name that you opened it with, overwriting the old file with any changes that you have made. If this was a new painting (file) it will ask for a name before saving. It's usually a good idea to use 'save' every 15 minutes or so when putting a lot of time into a project, because if your program crashes for any reason (power outage, gremlins, etc.) you would lose all of your work since you opened the painting file.
Hope that answers your question.
Give a man a fish and you feed him for a day - Teach a man to fish and he will sit in a boat and drink beer all day! Joe V.