A hot spring is a spring whose water is hot. There are hot springs all over the earth, on every continent and even in the oceans. The groundwater emerging from hot springs is heated by geothermal processes.
The water issuing from a hot spring is heated by geothermal heat, i.e., heat from the Earth's interior. In general, the temperature of rocks within the earth increases with depth. The rate of temperature increase with depth is known as the geothermal gradient. If water percolates deeply enough into the crust, it will be heated as it comes into contact with hot rocks. The water from hot springs in non-volcanic areas is heated in this manner.
In volcanic zones such as Yellowstone National Park, water may be heated by coming into contact with magma (molten rock). The high temperature gradient near magma may cause water to be heated enough that it boils or becomes superheated. If the water becomes so hot that it builds steam pressure and erupts in a jet above the surface of the Earth, it is called a geyser.