Gas unit heaters are ductless, self-contained, automatically controlled, vented, gas-burning appliances that distribute warmed air without the use of ducts. They have a fan that circulates the air and is typically used in commercial/residential garage settings.
Much like it sounds, "radiant/infrared" heat is simply heating that radiates out from a central source - in this case, from a radiant/infrared heater unit. A radiant/infrared heater will work much more efficiently when it is placed in areas near to the people or objects that need to stay warm. Here, it doesn't matter as much if the room is open or enclosed, since it is the individual persons or objects that are being heated rather than the air itself.
Also, since radiant/infrared heaters don't rely on blown air to heat a space and thus will cause no drafts.
Finally, radiant/infrared heaters tend to be ideal choices for rooms that serve a specific purpose, are smaller in size and may go unused for long periods of time during the day or night. Garages and/or storage sheds that need to be temperature controlled, patios and workshops are also all good examples of rooms where radiant heat can be an excellent, energy-efficient choice.