Save: File>Save allows you to save the photo you’re working on to view or work on later. You can specify a file name, the type of document you want to save it as, and where you want to save it on your hard drive.
Save As: File>Save As works almost the same as Save, but allows you to save a copy of what you’re working on with a different file name or document extension. For example: you have two photos of a shirt. You have made one shirt orange and the other shirt red. Rather than saving the orange one and then saving over it with the red one, you can specify the file names and save the same document once as orangeshirt.jpg and one as redshirt.jpg.
Save For Web: Save for web allows you to save a photo that will be best viewed on the internet. It will automatically set the file size to cut down on load time if you add it to a web page, and will also allow you to specify the quality of the photo, the size, and other options.