They are small birds that originate in America living across the whole continent, but they mostly inhabit tropic regions. Their flapping wings make a special humming sound and that's why they're called so. The hummingbirds are also rare due to their high frequency of wings beating. The highest number of wings beating per second was measured at 88/sec. Such a frequency helps them to hover in the air.
Another distinction from other birds is the sweet receptor that helps them find sweet nectar. The nectar serves as a source of energy.