A typical MHL adapter has an MHL connector on one end (for plugging into the phone), an HDMI port on the other (for connecting the phone to any HDMI display), and a micro-USB port on its side since the phone's own micro-USB is blocked when this adapter is used.

That micro-USB port on the side of the MHL adapter allows you to charge the phone while playing video to your HDMI display.

As of 2013, there are 4 types of MHL adapters

    The Galaxy SII, Note 1, and Galaxy Nexus (and more then 50 smartphones by other vendors) support MHL 1 with a 5-pin connector,
    The Galaxy SIII and Note 2 support a 11-pin version of the MHL 1,
    The HTC One is the first phone to support MHL 2 with the standard 5-pin connector.  It is backward compatible with MHL 1  5-pin adapters.
    The Galaxy S4 is the first phone to support MHL 2 with the Samsung 11-pin connector.  It is backward compatible with the GSIII/Note 2 adapters.

Version 2 of the MHL standard increases the maximum supported refresh rate from 30 to 60fps (at 1080p), and it also doubles the rate at which the MHL cable can charge the phone (when used in conjunction with a micro-USB cable).

The 11-pin connector allows the phone to power the MHL adapter itself, so using the micro-USB port on the side of the MHL adapter becomes optional.  As well, it extends the functionality of that micro-USB port beyond just charging - allowing it to be used as a USB-OTG port so you could connect e.g. a keyboard and a mouse.

Please note that all of these are considered standards conformant products in spite of the different connectors - the MHL standard does not specify a connector.  I have referred to 5-pin as 'standard' because it is what everyone used initially and everyone but Samsung still uses.  I consider it unfortunate that they did not standardize the connectors, or at least use a different name for the different connectors the way the USB and HDMI standards did (e.g. micro-USB, and mini-HDMI).