Actually, there are different types of sprinkler systems. In lots of residential homes, a personal protection type system are being installed. These systems use existing water pressure and lower flow heads. The heads are not interconnected and only release water if the temp is high enough at the individual heads. The purpose is not to extinguish the fire, rather give occupants more time to escape a burning building. These are very affordable and becoming quite popular. In fact, they are required in many areas now, especially for new construction and multi-unit rental properties. They are easy to install, especially in new construction as they use standard 1/2 inch PEX tubing and an inexpensive thermo heads.
These differ greatly from the traditional pressurized systems that are designed to trip all the heads if one is activated and supply a deluge of water to extinguish fires. These systems are normally monitored by a security company or local fire dept via phone lines. They use a control board, communications modems, and closed loop water supply systems. The high cost of this type of system is the reason they are rarely used in residential settings.