Usually not much, unless you refer to so-called fullface Moutain Bike helmets with a "bumper" that protrudes in front of the chin to protect it.
Usually a moutain bike helmet has a visor which sometimes can be removed. Reasoning on having a visor? Mountain Biking usually meant going into the mountains on a single track and going in and out of the shadows from trees.
So this means usually road bikes don't have visors. Road bikes put your body posture more prone so if you had a visor your vision would be partially blocked.
Road helmets also more aerodynamically streamlined. As far as vents go, they usually have more but are smaller which helps with speed again.
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The difference is the purpose they are used for. Ski goggles are obviously used for skiing while mountain bike goggles are used for cycling. Ski goggles have features and technology to withstand extreme wind, snow, and sun reflection during skiing while mountain bike goggles are not made for extreme situations such as strong wind, snow, and sun reflection in skiing.