Hy.dro.ther.mal adj. \ˌhī-drə-ˈthər-məl\ : of or relating to hot water –used especially of the formation of minerals by hot solutions rising from a cooling magma1.

Hydrothermal vents are peculiar geologic features found in many parts of the world: at the bottom of the ocean, near volcanic regions, even on celestial bodies in space. What they have in common is that they’re all found in areas that are volcanically active, that is, in zones where there is magma close to the surface of the crust. Molten rock seeps up through fissures in the crust until it reaches near the surface layers. It heats up groundwater or cold oceanic water immediately contacting it, either turning the water into steam or superheating it (above 700 degrees Fahrenheit!) while infusing the water with minerals found in the magma.

The geysers at Yellowstone National Park, located in the U.S. states of Montana, Wyoming, and Idaho, are a famous example of a hydrothermal vent system. In the ocean, regions around rift zones, such as the Mid-Atlantic ridge, faults, and subduction zones (where oceanic crust is pushed under another plate), are the most common places where hydrothermal vents are found.

Underwater hydrothermal vents are mysterious locales where primitive
life forms such as tube worms and pale ghost crabs thrive, even under
extreme temperatures, pressure, and the absence of sunlight. A delicate
balance of geologic chemicals and heat sustains micro-ecosystems
immediately surrounding the vents in a process called chemosynthesis.
Microbes around the hydrothermal vents make use of the compounds mixed
into the hot water, such as hydrogen sulfide, a substance toxic to
humans, and convert them into energy that can be used by organisms
further down the food chain, such as tube worms. Minerals and metals are
also deposited alongside the vents and have shown some potential as
mining sites for copper, lead, zinc, iron, silver, and gold2.

Sources:
1. Merriam-Webster: Hydrothermal
2. National Geographic Education: Ocean Vent

http://www.botos.com/marine/plumecartoon.jpg
http://faculty.cascadia.edu/jvanleer/astro%20sum01/Hyrothremal%20Vent%20Final/hydrothermal-vent.jpg