Picture something like an X-10 appliance module, but with two screw terminals. Short the terminals, and the internal relay closes and energizes whatever load is plugged in.
The module contains an internal class-2 power source, so nearly anything can be connected connected to the low-voltage, low-current terminals: a magnetic reed switch, a float switch, a pendant switch, a floor mat switch, whatever.
Quite possibly overkill for most applications, but such a beast IS manufactured for interfacing 120VAC wiring to intrinsically safe switch loops in hazardous areas. 3 screw terminals on the 120 side (line,load, neutral), and 2 terminals on the control side that energize the load when shorted together. Here's one manufacturer's version,which I have installed a few of:
http://www.gemssensors.com/SpecTemplateStandard.asp?nProductGroupID=152
