Save vs Save As
When saving your documents, you are given the two choices, Save and Save As. Although these two functions have the same goal, there are minor differences in how they achieve their goal. Save automatically writes the changes to the file that it was last saved to or to the file from which it was opened. Save As does not care about which the history of the file. It would always open a new dialog where the user would enter the filename he wishes to use and the format.
Save is very convenient as it allows you to save your file with a simple click of a button or a quick hotkey press. Little else happens on screen, allowing you to proceed with your work without any interruption. The dialog in Save As is another extra step that you do not really need when you are saving to the same file. Having to deal with a dialog often breaks concentration and can be a bit disruptive to your chain of thought. For this reason, Save is the most preferred between the two when all you want to do is to commit your changes to the file.
Despite being a bit more cumbersome, Save As does have its own use that is very important. Save As allows the user to save to another file without needing to commit the changes to the file that it was from. It also allows the user to change the format from one to another. This can be important when the files can be in many formats and the person you need to submit it to expect another format than what you already have.
Having stated all the differences for Save and Save As, it is also good to note that when working with a new file that has not been saved yet, Save would bring up a dialog just like Save As. This is because it does not know where to save the file yet and the only way to save it would be via Save As.
Summary:
Save writes all the changes to the same file while Save As always brings up a dialog
Save is quick and easy while Save As adds another step and can take a couple of seconds more
Save does not allow you to save to another file or format while Save As does
If a file has not been saved yet, Save functions just like Save As