There are two basic types of heating and air conditioning systems, putting aside the differences in cost, brands, and efficiency. Probably the most common is the split system. Less common, but still quite normal is the package unit.

      The split system consists of two parts. Outside are the condensing unit (solely for AC) or the heat pump (which provides heat and AC), but look and function basically the same way. The outside section is connected with copper pipes to the inside portion where you find the evaporator coil for AC, (and heat in the case of a heat pump), the blower, that pushes the air thru the ductwork, and possibly an oil or gas fired furnace to provide heat.

      The package unit is situated outside of your house, usually adjacent to the crawl space, and contains the condenser coil, evaporator coil, the blower and the heater; all contained in one package, hence the name. The connection to the ductwork is usually directly thru the foundation, into the crawl space.   

      The choice of which type of system to use is usually made based on practical matters such as accessibility to space needed for ductwork, electrical requirements, drainage demands, and minimizing disruption to interior spaces. Package units are more prevalent in retrofits in houses that were not built with a central system.