Water-to-air geothermal heat pumps heat and cool air, for forced-air ducting systems. In the United States, most homes use forced-air ducting distribution systems, so water-to-air units are the standard type used.
A water-to-air unit has 1 water coil, and 1 air coil. The water coil is connected to the earth loop, and the air coil is connected to your air ducting. Air from inside your home is circulated through this air coil to be heated in the winter, or cooled in the summer.