The Equinox uses a fuel cell that runs hydrogen across membranes, which combine it with oxygen from the air to create water. Each water molecule that's created pops loose one unneeded electron. That flow of electrons is what powers the car.
But creating pure hydrogen - it's usually separated from water or natural gas - requires power itself. That power usually comes from electricity, so some critics say hydrogen power won't do away with the vehicle's impact on the environment and will instead just shift the problem elsewhere.
Hydrogen fuel cell supporters counter that fuel cells are much more efficient than internal combustion engines, so less hydrogen is needed than an equivalent amount of gasoline. Also, electrical generators are much more efficient, so getting power from electricity - whether its by plugging into an outlet or separating hydrogen - is a big improvement over internal combustion.
Finally, hydrogen power would at least take cars out of the environmental equation, simplifying efforts to reduce emissions.