| The Oxford English Dictionary says that mineral water was "originally water found in nature impregnated with some mineral substance, usually, such as is used medicinally," but notes that the term is sometimes "applied also to artificial imitations of natural mineral waters, e.g. soda water, seltzer water; and subsequently extended to include other effervescent drinks." The U.S. Food and Drug Administration now defines mineral water as "bottled water with at least 250 parts per million in total dissolved solids" from a source "tapped at one or more bore holes or springs, originating from a geologically and physically protected underground water source," and most other countries that regulate mineral water also specify that the minerals in it can't be added during packaging.
The original point of mineral water was that it was supposed to be good for one's health; the OED records the phrase back to the 16th century, with an 1831 medical book saying it applied to waters that "contain in solution one or more foreign substances in sufficient quantity to exercise a more or less marked action on the animal economy." However, the FDA forbids any health claims to be made for mineral water because they are still "unproven." This may be partly because there are no standards for what specific minerals have to be in the water; the European site mineralwater.org lists possible components of just the few brands it reviews as: arsenic, boron dioxide, bromine, calcium, calcium carbonate, chloride, cobalt, carbon dioxide , chromium, copper, fluoride, germanium, hydrogen carbonate, iodine, iron, lithium, magnesium, manganese, nitrate, potassium, rubidium, silica sodium, strontium, sulphate and zinc. You'd have to read the ingredients carefully to find out which of these a particular brand contains.
The use of mineral water dates back to ancient times, although in the past it was just as common to bathe in mineral springs as to drink the water. (In fact the practice of immersing oneself in water from the hot mineral springs in the town of Bath in England gave the town its name.) Hydrotherapy has gone in and out of fashion, but the drinking of mineral water seemed to become more popular when it was no longer necessary to travel to the water's source to drink it.
Sources:
http://dictionary.oed.com/cgi/entry/00147132
http://www.bottledwaterweb.com/regulations.html
http://www.fda.gov/bbs/topics/NEWS/NEW00333.html
http://www.pmgeiser.ch/cgi-bin/disp?mineral/content/index.htm
http://www.modlife.com/MLG/editor/history.html
http://www.deremilitari.org/gestastephani.htm
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