You may want to clarify that, in North America, a geyser is always a natural geologic vent from which hot water and steam periodically vents. However in some countries, it refers to a device which heats water as the water passes through it, what would in North America be called a Tankless Water Heater.

Natural geysers are hot springs that intermittently throw up a jet of hot water, stream, etc. The earth's crust has cracks and vents. Water occupies these and gets heated due to geothermal energy.

Due to heating, water turns into steam and pressure increases. In this case, if the opening of the vent/ crack is narrow, it erupts like a volcano, otherwise it flows like a stream. The 'Old Faithful' geyser at Yellowstone National Park in the US is one of the most famous.

A hot spring in which water intermittently boils, sending a tall column of water and steam into the air.

old faithful, in Yellowstone park.