Let's clarify the difference between the commands "Save" and "Save As..." for those who may not know. I'll be using a Microsoft Word doc as an example, though these rules apply to just about every program.

When we create a new document within Word, that document won't get saved to the hard drive until we tell Word to do so. Simply click "File" then "Save". Word will then ask us o title the file, and where we want to save it. If we continue on modifying our document we

need to tell Word to save it again. Clicking "File" then "Save" will cause Word to overwrite the older file with our current one, without prompting us for anything. This is a good thing, otherwise we'd have a new file every time we saved, which would lead to a lot of clutter.

But let's say we do want to save our document as a new file, as to not overwrite the existing file.  That's what the appropriately titled "Save As..." is for. When we click on it, Word will then prompt us for the new file name, and where we want to save it (just like when we saved our document for the first time). This can be useful for holding onto multiple versions of a document. So if you decide you don't like some changes you made, you can revert back to an older copy. 

When saving a document for the first time the commands "Save" and "Save As..." perform the exact same function. Once the file has been written to the hard drive "Save" acts like an update button. "Save As...", on the other hand, will allow you to create a whole new file with a new title.

A quick tip: use the keyboard shortcut Control+S instead of using the mouse to click "File" then "Save". It's much quicker, and you'll get into the habit of saving your document more often.