Muggles' Guide to Harry Potter/Magic/Deluminator

Overview
The Deluminator, also referred to as a "Put-Outer", is a small magical device that looks like a cigarette lighter. It captures and stores a light source, like a streetlight, and can later be used to restore the light to its original place.

Extended Description
The Deluminator was used by Professor Dumbledore in Harry Potter and the Philosopher's Stone to "put out" the surrounding streetlights to make it harder to see the proceedings while Harry Potter was being delivered to the Dursleys. Once the wizards had left Privet Drive, Dumbledore released the lights from the Deluminator to restore the street to its normal night-time illumination. We note that the author refers to it three times as a Put-Outer; Dumbledore does not name it while he is using it.

It was later used in Harry Potter and the Order of the Phoenix by Alastor Moody to darken Grimmauld Place while the Advance Guard was landing in the square. Moody again restored the streetlights once all the wizards were inside the house, before closing the front door. The author here twice refers to it as a "Put-Outer", though neither Harry nor Moody uses that term.

Again, the Deluminator appears in Harry Potter and The Deathly Hallows. First named in Dumbledore's will, it is given by the Minister of Magic to Ron once the Ministry has duly examined it. Ron later discovers that, as well as being able to remove and replace light, the Deluminator has the ability to track Harry and Hermione, despite their magical shields. It seems to key on the use of his name, picking up their conversation from the point at which they mention Ron. Having started tracking their conversation, it seems to be able to provide some sort of transport to their vicinity; it is never entirely clear how this is managed, exactly, but it produces a ball of light which, when it contacts Ron's chest, allows him to Apparate to Harry and Hermione's vicinity. Finally, in the basement of Malfoy Manor, Ron discovers that captured lights will simply float in the air, if they are released in a place distant from their original sources.

Analysis
Dumbledore, as we can see by even a casual inspection of his office, is very fond of creating magical / mechanical hybrid devices. Given that Moody says he "borrowed" the Deluminator from Dumbledore, we can assume that it is a rare device in the Wizarding world; it is entirely possible that it is, in fact, a unique artifact, an invention of Dumbledore's.

In the real world the closest analogue to the Deluminator is a device called the TV-B-Gone, which works on most, if not all, TV sets by generating a sequence of "Off" codes. This has been documented to also jam the light sensors on some street lamps. A hypothetical variant of this which has yet to be further developed uses a hybrid frequency detector to resonate with the 30-80 kHz ballasts used on many CCFLs and high pressure sodium street lights, and generates an inverse signal that causes a false event in the controller causing the light to go out as if the tube had quenched. Such a device would need to generate field strengths of several tens of kV per metre to travel up the (typically iron) pole and interfere with the ballast. Neither of these devices, however, has the ability to recreate the captured light source in a separate location, as the Deluminator does in the cellars of Malfoy Manor.

Greater Picture
Magic is of course keyed to words; an incantation will not work, even with the correct wand motion and mental preparation, unless the key word is spoken or thought. A more specialized instance of this is the "taboo" placed on his own name by Voldemort; first mentioned in Harry Potter and the Deathly Hallows, this spell alerts Voldemort whenever anyone speaks his name, breaks many defensive spells, and summons Voldemort's minions to the place the name was spoken. If this spell was in use during Voldemort's first ascension to power, it would explain the widespread reluctance to speak his name. Clearly the conversation tracking ability of the Deluminator is very similar to this taboo, as the available effects are very similar, though benign.