It's a VHF radio system used for ambulances to speak with hospitals. It's cousin is the MED radio, which uses UHF frequencies to accomplish the same task.
Generally, you'll find that HEAR channels will have a number of ambulances/hosptials using them, will it's usually one ambulance/hospital per MED channel.
Some areas are seeing decreased use of these channels due to cell phones, and the availability of a dedicated channel on a TRS for an agency to speak to a hospital.