Muggles' Guide to Harry Potter/Magic/Portkey

Overview
A portkey is a common item used to transport anyone touching it to another pre-determined location, such as a Quidditch match. Typically portkeys will activate at a specific time, but apparently it is also possible to create an on-contact Portkey that activates immediately when touched, or an on-command Portkey that activates on demand. A Portkey can be any sort of object, most often something of no value: an old teakettle, for instance, a discarded boot, or the head of a broken statue. A Portkey cannot, we believe, be a human.

Extended Description
Portkeys appeared in Harry Potter and the Goblet of Fire first as a means of getting to a major event, specifically the Quidditch World Cup, where it was used to transport those members of the Weasley family and those members of the Diggory family who were unable to Apparate. It was later used to transport those same individuals back home. In this section, we learned that Portkeys would typically be unobtrusive items that would activate at a specific time, transporting all who were directly in physical contact with them to a specific other point.

A Portkey was also used to transport Harry and Cedric Diggory to the graveyard where Voldemort staged his return; in this application, the Portkey activated immediately upon contact, taking Harry and Cedric from the center of the third-task maze to the graveyard, and then later carrying Harry and Cedric's body from the graveyard to the edge of the third-task maze.

On two separate occasions in Harry Potter and the Order of the Phoenix, Professor Dumbledore is seen to create a Portkey, once to carry Harry and the Weasleys from his office to 12 Grimmauld Place, and later to carry Harry from the Ministry of Magic to Dumbledore's office. Both of these Portkeys seemed to be of the delayed-activation type, as in both cases Dumbledore counted down for the portkey to activate; in the first case, this was necessary because it was necessary to transport all of the Weasleys, along with Harry, to Grimmauld Place, in the second case it was because Dumbledore wanted to hand the Portkey to Harry. In the first case, though, it seems more likely that the Portkey was a command-activation type, because while Dumbledore did count down to the activation, he had made the Portkey before he had heard back from the portrait of Phineas Nigellus, and he could not have known exactly when it was to activate until Phineas Nigellus had reported back.

Analysis
When Dumbledore uses the Portus charm to create a Portkey in front of Cornelius Fudge in the Ministry of Magic, Fudge, then Minister for Magic, seems shocked that Dumbledore would so lightly create an "unauthorized Portkey". It appears that, because of their ability to deposit a person within areas that are protected from incursion (recall that both Hogwarts and 12 Grimmauld Place are protected by being made unmappable and by having spells to prevent Apparition), creation of Portkeys is controlled by the Ministry.

Apparition requires a firm knowledge of the destination to which you will be going; one must assume that creation of a Portkey requires at least that same understanding of the destination, probably even more. It is certain that the two Portkeys that Dumbledore creates are for destinations with which he is extremely familiar. It is interesting that, rather than create another Portkey for himself, Dumbledore returns to Hogwarts by way of the Floo Network, especially since Hogwarts attachment to the Floo network is supposed to be somewhat thin.

In Harry Potter and the Deathly Hallows, Mad-Eye Moody tells Harry that the area around Privet Drive is being monitored for use of the Floo Network, Apparition, and use of Portkeys, ostensibly for Harry's protection, but actually to monitor where he is taken if he leaves there. From this, we may gather that Portkey use can be monitored if not prevented.

Portkeys are used later in that same book to bring various groups from safe-houses scattered around the country to The Burrow. Harry and Hagrid, for example, catch a hairbrush from Ted Tonks' house. These Portkeys are timed; there is some discussion in the Tonks house about not missing the Portkey, and there is concern at the Weasley house because two prior Portkeys had already returned without the Order members who were to travel with them.

Greater Picture
It has been mentioned that one of the problems with writing a book about magic is the entire issue of limitations. If your hero can do literally anything by means of magic, then you have no story. We believe that the regulation of transportation, as shown here by Fudge's reaction to Dumbledore's creation of an "unauthorized Portkey", is a way of applying a judicial limitation to an otherwise over-extensive form of magical transportation. We have noted above that in the two instances where Dumbledore is seen to create Portkeys, the intended destination is unmappable and proof against most forms of magical access; Portkeys clearly can circumvent the barriers to use of Apparition and the Floo network that are present at Grimmauld Place and at Hogwarts. In the final book, we are told that the use of Portkeys is being monitored in Harry's neighborhood; it would seem that the Ministry is able to detect not only that a Portkey is being created, but also that it is being used. This may be the only thing that prevents Voldemort from using a Portkey to enter places ordinarily forbidden to him, like the Ministry.