Aero (or aerodynamic) simulations mimic the movement of gases. They are used to create smoke, mist, and steam effects. Combustion simulations mimic the burning of fuel, and are used for fire and explosion effects.
Note that in general, aero simulations are deterministic, meaning that separate simulation runs with the same parameters and inputs produce identical runs. However when memory is low, Bifrost might switch internally to a slower algorithm that uses less memory. This means that the results might be similar but slightly different if different amounts of memory are available between two simulation runs.