There are several systems of hydrants in the market.
Barrel hydrants: It is the typical hydrant, that you can usually find, which has the valves easily accessible. There are 2 types:
Wet barrel: They have the advantage of having independent valves in the same body, it is easy to install and repair, because the entire system is seen.
Dry barrel: They are suitable for cold weather areas. They have a buried valve, that you can open, from the top of the hydrant. This means that the water keeps under the ground, preventing it from freezing. The only problem with these hydrants is that if you want to repair them, you need to dig to discover it whole valve.
Underground hydrants: These hydrants are placed in traffic areas, where it is not possible to install a barrel hydrant. A typical example would be in a truck area, in which to meet the minimum distances, it is necessary to place the hydrant in the traffic way. In addition, they also offer freezing protection.