Since this actually means that existing S III units will leapfrog Android 4.2, you can imagine that quite a few new goodies are coming our way. Unfortunately, Samsung doesn't really subscribe into all that changelogs-type-business, so there's no official list, though the good folks over at SamMobile have compiled a pretty extensive list of what they've found to be changed. Perhaps most important are the cosmetic changes – the UI has been updated, and now appropriate the Galaxy S4/Note 3 theme elements – and the addition of garbage collection with fstrim support. TRIM helps a great deal when aging devices (like the S III) are concerned, as these that suffer poor storage I/O performance after a while. In other words, expect some tangible performance boost with this update. Check out the full list below.
It's likely that only a few of you have already received the update prompt just yet, however. You could try to force an update check manually, by going into Settings > More > About Device > Software update > Update, but you'll probably have to wait a while. Moreover, if you're locked to a carrier, it will take even longer as your service providers needs to do some work as well. Either way, you can keep track of which countries/carriers have started receiving the new firmware with SamMobile's ingenious tool right below.