Once the domain of the social elite, spas have emerged as trendy oases of calm for even the hoi polloi. They now no longer merely offer massages, steam wraps, and manicures, but rather a whole cosmopolitan medley of "treatments," including standard facials, acupuncture, kick-boxing, spiritually focused tai chi, and self-discovery hikes. There's something for everyone and (more importantly) every budget, from hour-long day-spa treatments to weeks-long immersions at destination spas in exotic settings.

With more than 12,000 spas in the U.S. alone, how do you choose? Be it a city-center sanctum of peace or a luxurious resort spa with sweeping ocean vistas, it helps to know the differences among the various kinds of spas and their menus of offerings.