save will save your current state
save as will allow you to change parameters
so if you had an open text or picture "save" would save exactly as it is right now, but save as would allow you to...
main difference is because with Save you can update your saved and opened document without Save as dialog in explorer but to do this document must be already saved on hard drive
When you select "Save" the system assumes you are saving to the same file name.
"Save-As" gives you a chance to save to a New File Name leaving the original file untouched by your most recent...