A repeater is a major part of amateur radio infrastructure that listens for a signal on one frequency and repeats it on another in real time. Many amateur repeaters operate between 144-148 MHz and 420-450 MHz, with fewer found within the 29MHz, 50 MHz, 220 MHz, 900 MHz and 1.2 GHz spectrum.
Radio signals can mostly only travel in "Line of Sight" at frequencies over over 20-30 MHz, so putting a repeater station as high as possible lets users with low power hand held radios communicate over greater distances and obstructions.
According to Repeaterbook.com, there are 772 repeaters as of September 2018 spread across different frequencies and bands of operation just in New York alone.
Digital repeaters, especially those that use DMR create more opportunity to reach someone since they are more easily linked to the internet for times when there are no "local only" people to talk to or potentially make a call during an emergency situation.
HVDN is focused on keeping track of the latest digital repeaters such as those that use DMR becuase the represent the largest emerging trend in the hobby today with the most potential for other add on functions such as artificial intelligence, machine learning, text messaging, location technology and much more.
Since amateur radio is a hobby, there are no commercially funded repeaters.
Equipment, time, site leasing and other expenses are shouldered by individuals and clubs that see repeaters as an investment so many people can further enjoy amateur radio.