Android 4.0 already has a feature called Android Beam, which works over NFC to transmit information such as contact details and browser pages from one mobile to another. But it doesn't use Wi-Fi Direct so it's impractical for larger files.
By combining NFC and Wi-Fi Direct, S Beam is capable of sending larger files between phones, such as images, videos and music tracks. The transfer is initiated by NFC and the actual file transfer is handled using Wi-Fi Direct. This means you can expect transfer speeds of up to 300MBps.
How to enable S Beam
You can activate both Android Beam and S Beam through the Settings app on your Galaxy S3. To do this, go to Settings > More settings, and you will see separate entries for both. Tap on each of them to ensure that they are activated.
You will need to enable NFC in the same settings window as well, as this allows the phone to transfer information with other NFC-enabled devices.
How to transfer files using S Beam
Transferring a file between two phones couldn't be simpler. You simply navigate to the image, video or music track that you want to send and hold the back of your phone against the back of another S Beam-capable device. Your phone will tell you to 'tap to beam', and the file transfer will begin.
As the transfer uses Wi-Fi Direct, you can then take the phones away from each other and the transfer will continue uninterrupted. You don't need to be connected to the same network for this -- it all happens automatically between the phones.
At the moment, S Beam is only available on the Galaxy S3, although as Samsung continues to release more Android 4.0 devices, you will be able to transfer files in this way between more models. You can use the Android Beam functionality with any phone running Android 4.0 or later that has NFC capabilities.