hydropower:
what it is;
ydro power is used to generate electricity. Eg Boulder dam
Once the electricity is generated, it is mixed in with the rest of the power grid.
So users of electricity can't tell if their electricity came from a hydroelectric power plant, a nuclear power plant, or a wind turbine power plant.
Things powered by electricity: everything around you, TV, radio, computer, street lights, toaster, etc.
A more general answer would be to say that hydropower produces energy by converting the kinetic energy of falling water into other types of energy.
Electricity has already been mentioned, but hydropower can also be used to run mechanical structures such as in water mills to grind food, cut logs, etc. Ultimately this is how electricity is produced by turning a mechanical turbine attached to a magnet.
I assume what you're actually asking is what makes it more useful than conventional fossil-fueled based electric sources. Hydroeletric plants produce energy by tapping into the kinetic (and at times potential) energy of water. An overly simplified example is this: imagine there is a lake at an elevation of 1000ft. The lake is in an area that rains a lot and keeps the lake full. It rains so much in the lake that there is a river going from the lake that eventually empties out into the ocean. If you put a turbine in that water, the river will push the turbines as it flows. That kinetic energy is then converted to electricity (copper moving through a magnetic field; the explanation of how electric is produced in that manner is beyond the scope of this question), and the electricity is then sold to consumers. One complaint about hydroelectricity is that sometimes it can have a negative effect on the localized environment by upsetting the natural flow and status of the water source it's