if you just save something like a word document and you just click save it wont save the word document in the location you want to save it to where as if you click save as you choose a specific location to save a document.
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...
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...
When you work with open document which already have been saved, you can press Save and this will save changes without Save As dialog in Explorer, any time you press Save As will open Save As dialog...