Rooting your device gives you all-powerful administrative access to your phone and its system settings. Of course, with such power comes great responsibility, and sometimes that responsibility becomes to burdensome or just plain unnecessary.
There are many reasons for wanting to un-root your Android phone. If you've bought a phone from a friend or online and it came rooted, you may want to revert to stock, or if you're having problems and want to avoid breaching your warranty requirements, you'll want to the device back to its factory settings and systems. Maybe you just changed your mind about waiting for Jelly Bean to really hit your S3.
Whatever your reason is, it's not too difficult to get your Samsung Galaxy S III back to its original, out-of-the box Ice Cream Sandwich settings.
As with all things root-related, make sure to back up your Galaxy S3 before proceeding. You may want to download the stock firmware and the free Triangle Away app (or paid version) before proceeding with the un-rooting process. Triangle Away allows you to restore your flash counter to zero, in order to avoid losing your warranty (flashing your device voids the warranty).
For those who don't like watching 20+ minute video guides, you can check out the full written description of the process over at Totallydubbed.