My understanding (possibly flawed) is that SAVE FOR WEB discards all information not absolutly nessesary for posting a web image. As pointed out in an earlier post, you also have more control over the compression ratio. I experimented with a small image and swung the compression ratio into the BAD area... VERY UGLY!  Try it, you will most likely learn more from expermitation than you did by reading this post LOL!

"Dance like nobody's watching; love like you've never been hurt. Sing like nobody's listening; live like it's heaven on earth."
  --  Mark Twain --