"Developed" versus "wild" hot springs, and what's a spa?

The term "hot spring" means different things to different people, and it's a good idea to know just what manner of hot spring you're bound for at a destination, because it may be something quite different from what you're expecting. In English the term is used more or less interchangeably for "wild" springs, where the water emerges directly from the ground into a natural catchment that can be used for bathing, and "developed" springs, which exploit the spring through construction of man-made artifacts such as pools and bath houses. "Wild" springs and "developed" springs can differ so much, and in so many ways, that the visitor expecting one and getting the other may not enjoy the experience very much. 
Some examples:

Wild springs are often (although not always) on public land or otherwise accessible to the public without charge, while developed springs are almost invariably intended by the developer to make a profit, and hence will charge (and be in a legal position to demand) an admission fee.

You can't count on creature comforts at a wild spring; you may have to sit on a rock at water's edge to doff your clothing, and pre-entry showers are pretty well out of the question, let alone amenities like poolside drinks that a developed spring may offer. On the other hand, wild springs are generally open-air and take you "back to nature" in ways that a developed spring may not.

At a wild spring, water temperature is purely on an as-is basis; the pool where you bathe will be at a temperature that's regulated solely by the relative proportions of water from the spring and meteoric (surface) water that the terrain imposes. As a consequence, water at wild springs can be uncomfortably, or even dangerously, hot. Commercial operators of developed springs will generally ensure that the water temperature is appropriate (sometimes offering several choices of temperature in different pools) through dilution of the spring's effluent with water from the commercial supply or other sources. This distinction is particularly important; the bather used to "tame" water from a commercial spring who wades directly into a seething-hot wild spring can receive a painful, or even fatal, surprise.

Hot-spring water is usually fairly safe from the standpoint of carrying disease-causing organisms, but some is not (see below under "Stay healthy"), and the surface water that cools a scalding spring to usable temperatures will be prone to the same bugs and pathogens as any other surface water. Operators of developed springs may (or may not) take steps to disinfect the water, but at a wild spring, you're obviously on your own.

Note, incidentally, that a "developed" hot spring is not necessarily a commercial hot spring, i.e., one that has been developed for profit-making purposes. The distinction can be important in countries and regions where the political/economic system allows for both for-profit and public-interest/non-profit/governmental development; regulations for doing the developing will often differ between the two cases, as will the resulting amenities, access, etc. For example, as a general rule, springs in the United States that have been developed by government will have fewer amenities, but also lower admission fees, than for-profit developments. In Japan, many hot springs in rural locations are maintained by the local government and are open to the public for free, and even expensive spa resort towns usually have at least one public bath open to all for a token fee.

Spas

There is a difference between a hot spring and a spa. The latter term denotes either a pleasantly warm tub of water (not necessarily originating in a hot spring) suitable for bathing for medicinal and recreational purposes, or the -- sometimes incredibly elaborate, luxurious, and expensive -- resorts where such tubs can be found, which incorporate massage, body wraps, and so on. Not every spa is based on a hot spring (many, perhaps most, simply heat meteoric water to the desired temperature); not every developed hot spring has spa-like amenities.