Radiant heat is a system in which the heat energy is transferred from the radiant directly to the charbroiler's grate. In a radiant heated charbroiler the radiant grates are generally made of cast iron and typically have angles to deflect the heat giving you more control over the temperature.
Vulcan offers certain units with infrared energy to heat the grates. This type of charbroiler normally costs more but cooks faster and uses less energy than standard charbroilers.
There are many common misuses of the word infrared when speaking about a charbroiler. Generally speaking, most of your over-fired charbroilers (flame is above the product and heat is pushed down using air) are infrared heat broilers.