e-hookah is a marketing term, especially for non-nicotine flavored units. these are typically low-buck ego type units, though there is at least one unit that is actually designed to look like a hookah hose handle.
as for the technological relationship between ecigs and hookah, ecigs are in fact basically small electronic hookahs. hookahs use heating to vaporize/boil off a flavored juice solution containing glycerine that is soaked into a susbstrate. in the case of hookah the substrate is tobacco, which is what provides the nicotine to the "smoke".