Natural Gas (NG) and Propane (LP) are the two most common gases used in gas grills. While similar in many respects, there are some differences in the gases and some highly important differences in the construction of NG and LP gas grills.
Natural gas, hence the name, is collected directly from the ground. Mostly methane, the gas is extracted from a drill rig piped into the ground and stored. Natural gas contains less energy than propane does, per the same cubic volume, and is stored and shipped to the home in a gaseous state compared to propane, which is kept in a liquid state, "liquefied petroleum gas," or LPG. This is the main reason why NG grills are built differently than LP grills. Propane is a byproduct of petroleum refining and trace amounts can be removed from natural gas drilling. For this reason, propane is far more potent than natural gas and contains more than twice the energy per the same cubic volume.
The pressure regulators in gas grills are the most differentiated between NG and LP grills. Ng contains less energy, and thus is less pressurized. For this reason, the gas orifices are larger in natural gas grills. The differences in pressure, gas state, and contained energy also account for the differences in the burners of a NG grill. NG burners have larger holes than propane burners so more gas can flow into the combustion area, as well as the burner air shutters, which increase or decrease the amount of air intake into the combustion area. The greater energy content of propane, as well as its liquefied form, accounts for the different construction between propane and natural gas grills. The gas orifices, where the gas leaves the fuel tank, are smaller in propane grills to account for the increased pressure and energy output. They are also configured to turn the liquid propane into a gas. The burners also have smaller holes leading to the combustion area, as well as burner air shutters, which allow air into the combustion area to mix with the gas before it's ignited. You should always know what kind of gas grill you have before operating. Where there are ways to convert natural gas grill to a propane grill, and vice versa, it is recommended that a licensed professional do the job since so many parts needs converting.