While the Samsung Galaxy S III/GT-I9300 is an Android 4.0 device, Samsung fine-tuned it a little bit and replaced some of the original Android apps, like Messaging and E-Mail, with its own TouchWiz 4.0-styled apps. In addition to original Google Mobile services like Gmail, Google Maps, Google Play Android Market, the Galaxy S III features apps like AllShare Play, which is a DLNA client; ChatON, Samsung's own kind of WhatsApp; a redesigned Contacts app; a FM Radio app; Gallery, for photos and videos; Game Hub, which allows to play and download games; a redesigned Messaging app, for SMS and MMS; More Services, Samsung's own app store; Music Hub, which is not live yet; Music Player, a pretty decent music player; My files, a file explorer and a redesigned Phone app, which even allows to do regular UMTS video calls.
Furthermore, Samsung has added S Memo, which allows to draw and write memos; S Planner, Samsung own calendar app; S Suggest, another app which allows to download apps from Samsung's own market; S Voice, which allows to interact with the Galaxy S III through voice recognition; Samsung Apps, yet another Samsung app market; Video Player, which is used to playback videos and can show videos as an overlay above other apps as well as featuring video trim functionalities and Video Hub, which allows to rent movies.
While in the past, Samsung was pretty often blamed for its bloated TouchWiz user interface, TouchWiz 4.0 is much lighter and nicer integrated, not bothering (too much) anymore. As a matter of fact, Samsung added some very useful features, like the video overlay which allows to playback videos while still being able to send test messages, E-Mails, etc. as well as the Music player is pretty powerful. And it can never hurt to have a file explorer available. Other apps, like S Planner, are still a little bit too playful and calling a simple calendar "S Planner" doesn't make too much sense. In this case it's questionable why the Music player isn't called S Music.
DISCLAIMER: The device above is pre-release hardware which came without any accessories. The firmware is close to the final release hardware.
Cheers ~ Arne