We’re starting to see devices that can output video over USB. These include phones, tablets and Chromebooks.
This is a good idea because it’s one less connector and the associated cables — rather than having a device with both USB and either Mini DisplayPort or HDMI ports, let’s make USB more capable.
In either case, since your monitor or TV most likely doesn’t support these new standards, you’ll most likely use an adapter, plugging your tablet or other device into the adapter, and plugging the adapter into your TV.
SlimPort is more flexible, outputting to either HDMI or DisplayPort , while MHL outputs only to HDMI.
Unfortunately, MHL adapters require power, which they draw from your phone, draining it, beyond the drain caused by playing video continuously. You can choose to separately power the MHL adapter, but that means your adapter now has three cables — input from your phone, output to the TV, and power. This is a mess.
SlimPort is better — the adapters don’t require power. But only if you’re outputting to HDMI. They do require power to output to DisplayPort.
Both SlimPort and MHL support 1080p video at 60 frames per second. The next version of MHL, which is MHL 3, ups the resolution to 4K, but at only 30 frames per second.
MHL 3 can also supply 10W of power to the tablet to charge while playing video, while SlimPort is limited to the typical USB power of 4.5W.
I don’t know about the data channel. The underlying protocols, DisplayPort and HDMI, both support a data channel. DisplayPort supports either a 1 mbps or a 720 mbps channel, while HDMI supports a 100 mbps one. Since SlimPort and MHL ultimately just carry a DisplayPort or HDMI signal, they should be able to support the data channel, though I don’t know if that is the case.
Why does this stuff have to be so complex, even for a geek? Why couldn’t all the companies agree on a single spec? More practically, given the existence of two standards, the best outcome would be for each of them to copy the desirable features of the other, so users have an easier time choosing between SlimPort and MHL.
 And older standards like DVI and (god forbid) VGA.