ND Geek is correct, but the 11 pin connector on the Galaxy SIII and any new cell phone/tablet/android device using this new connector can go further.
The OTG function is simultaneous with the MHL and the normal 5 pins are the OTG pins.
OTG hosts can, indeed, power USB downstream devices, but lack current for more than mice and keyboards (or memory sticks and cameras).
A powered HUB and an OTG cable with a Micro-B on one end and a Micro-A/B on the other end (the Host end) is needed to use a HUB. With this and a power box with 2A output can allow use of hard drives and CD/DVD players. The MHL connection is brought in through the extra 6 pins on the connector (only 3 of which are used for MHL).
The ability to charge the phone at the same time as outputting MHL and OTG requires 8 of the 11 pins and Samsung is being cagey about what those other 3 pins are for. In the end, the traditional 5 pins are for the OTG and that explains why the SIII is not backwards compatible with other MHL adapters or devices. This does mean that the 11 pin devices will support OTG natively and be backwards compatible. Hmm... 8 pins... sounds familiar, somehow.