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Helmets come into 2 basic configurations: Full face and open. Both have their advantages/disadvantages.
'Full face helmets are similar to dirtcross/motorcycle helmets with a rigid chin/jaw protection whereas open face helmets don't. On the other hand the more skateboard style 'open face' provides better overall visibility which is important if you are looking up most of the time.
If you're buggying go for the full face, if you're boarding try the open style. Either way its better have one than none at all.