HMDI is a single direction interface that is it is either an input or an output but cannot be both, so a TV has a HDMI input only interface and a Mac if it has an HDMI port is an output only interface. Therefore you cannot plug a video camera in to a Macs HDMI port and view the camera on the Mac screen.
Thunderbolt is however a bi-directional interface it can both send and receive so you can both connect it via a HDMI output adapter to a TV for example, or via a different HDMI input adapter capture the output of a video camera. These Thunderbolt to HDMI adapters are again only able to operate in a single direction because that is the limitation of HDMI. After all you cannot output from a TV via its HDMI port either.
iMacs of various models have in the past offered a 'Target Display' mode where either the Mini Displayport or Thunderbolt port can be used to receive a video signal as well as the normal output mode. Remember this is because Thunderbolt and Mini Displayport work in both directions but HDMI does not.
For some older iMacs you could also get a special Mini Displayport to HDMI input adapter from Kanex. See http://www.kanex.com/xd this does not work on newer Thunderbolt iMacs. You can connect two Macs together and use the iMac via a Mini Displayport cable to use the iMac as a display or for a newer Thunderbolt iMac via Thunderbolt. See Use your iMac as a display with Target Display Mode - Apple Support
As I indicated in my previous reply you can get various Thunderbolt to HDMI input interfaces such as the BlackMagicDesign Intensity or the Elgato Game Capture.
