A chiller is the central plant machine that removes heat from a loop of circulating water; that chilled water is then piped throughout a building to air handlers and fan coils that cool the air. Large offices, hospitals, schools, and high-rises use chilled-water systems because one central plant scales better than dozens of rooftop units.
The defining choice is how the chiller sheds the heat it collects. An air-cooled chiller rejects heat directly to outdoor air through fans — simpler, no cooling tower, lower install cost, and rated in EER. A water-cooled chiller rejects heat into a separate condenser-water loop served by a cooling tower; the U.S. Department of Energy notes water-cooled units are substantially more efficient, which is why they're chosen for larger buildings where the efficiency gain justifies the cost and maintenance of a tower. Water-cooled efficiency is measured in kW per ton (lower is better); both types are rated to AHRI Standard 550/590.
For an owner, chiller work is a different tier of contractor from rooftop service. Low-pressure centrifugal chillers require a technician with EPA 608 Type III certification, and the minimum efficiency for new equipment is set by ASHRAE 90.1. The honest market caveat: genuine chiller specialists are thinner on the ground than rooftop contractors, so in many smaller cities the credible chiller pool is limited — worth confirming a provider has actual chiller-plant experience, not just a willingness to quote one.