The image size settings allow you to resize an image to specific pixel dimensions or to a percentage of the original size. This can be handy if you forget to scale down an image before entering the SfW dialog. Or, if you can't quite squeeze the file size down to what you need using the optimization settings alone, you can often reduce the size even more by reducing the image size. These options are fairly straightforward and well documented so I won't go into detail here, but I have provided a screen shot of the various controls to the right.
Preview in Browser
At the bottom of the Save for Web dialog you'll see a browser icon and a drop down menu button. Clicking the browser icon opens a page in your default Web browser with a preview of the optimized image along with the format, dimensions, size, settings, and the HTML code to display the image. Clicking the arrow next to the browser icon allows you to choose any of your configured browsers or another browser. Choose "Edit List..." is you want to configure a browser to appear in the list.
Edit In ImageReady
The Edit In ImageReady button is also available if you need to switch into ImageReady for more advanced editing of Web graphics.
One important thing to know about Save for Web is that your image will always be exported with a resolution of 72 ppi, and it will strip out the Exif information in your digital camera photos. So if you need to retain the print size and resolution or Exif information, you shouldn't use Save for Web. Of course, you can always keep a working copy of your original file in Photoshop (PSD) or another format, and then save a Web version using the Save for Web command. This is a good practice to get into in general because you never know when you'll need to go back to that original.