The Water Source
The water for springs comes from underground sources called aquifers. Aquifers are most commonly inside permeable rock, or underground layers of materials like sand, clay, and gravel. These substances function as a sponge, soaking up water that seeps down into them. Less common are aquifers made of free bodies of water, but these do happen in areas where eroded limestone forms karsts.
A spring is simply a place where the water from an aquifer flows out of the ground naturally. In this respect, it is useful to think of a spring as a naturally occurring well. In a well, one must dig into the ground to reach the aquifer and tap the water supply. In springs, those conditions are already present and on the surface.
Some springs only flow during particular times of year. For example, an aquifer that is highly reliant on seasonal rainfall or snow-melt to feed it may not be able to sustain a spring all year around. There would still be water in the aquifer, but not enough to sustain the outflow of the spring.
Minerals and Hot Springs
Because aquifers are often made of stone, water passing through them will pick up minerals from local deposits. That is how mineral water is created. Because not all aquifers and mineral deposits are the same, the water from a particular mineral spring may have its own unique taste. Some aquifers are located near sources of geothermal activity, which heats the water. If that hot water feeds a spring, the result is a hot spring.