There are bicycle helmets which only cover the top of the head and don't cover the neck. There are also helmets that set farther down and cover the neck and give more support.
The traditional elongated helmet is called a road helmet. The more rounded one is called a commuter helmet. A youth helmet with vents (a mini road helmet) is made for riders who are between 10 and 15. Child helmets are for children ages 5-10 and toddler helmets are for younger.
There are many bicycle helmets out in the market. You have the option of road helmets, mountain, commuter, BMX, skate, children's, infants, and womens helmets.
You have the time trial helmet designed for hyper speed but not so effective on absorbing shocks, you have the mountain helmet for mountain and road accidents which is cheaper.
Road helmets for the road. Skate or BMX helmets for boarding and biking. Mountain helmets designed for mountain protection. Chrono for aerodynamic speed and MANY others.