Muggles' Guide to Harry Potter/Magic/Disappearing Cabinets

Overview
Disappearing Cabinets or Vanishing Cabinets are cabinets into which you put things, and the things then disappear, normally reappearing somewhere else.

Extended Description
We first see a Vanishing Cabinet, though it is not identified as such, in chapter 4, At Flourish And Blotts of Harry Potter and the Chamber of Secrets: Harry has ended up in Borgin and Burkes after a slight mishap with Floo powder, and has to hide to avoid Draco Malfoy and his father. The place he chooses to hide is "a large black cabinet".

Another Vanishing Cabinet appears in chapter 8, The Deathday Party, of the same book. Here, Nearly Headless Nick has convinced Peeves to drop a large black and gold cabinet just above Filch's office, in hopes of distracting Filch from punishing Harry for some infraction. Filch identifies the artifact at that time as "a very valuable Vanishing Cabinet".

Vanishing cabinets also form a major plot point in the sixth book of the series. As that constitutes a spoiler, that information is in the Greater Picture section, following a spoiler warning.

Analysis
It is interesting that Vanishing Cabinets first appear in book 2, and provide a major plot point in book 6. As vanishing cabinets are a classic part of many stage-"magic" illusionist acts, it's not surprising that they would be part of a world that uses significant magic; what we can't know is whether the plot device these cabinets represent was in fact at least partially worked out in book 2 where these cabinets are introduced, or whether their existence and mention in book 2 triggered their use as the plot device in book 6. We rather suspect preplanning, but we have no way to be certain at this point.

Greater Picture
Apparently the above two cabinets are linked. After repairing the Hogwarts cabinet, Malfoy uses the cabinets to allow Death Eaters into Hogwarts in his 6th year. We will note that there's significant foreshadowing in that book; the cabinet in Borgin and Burkes is specifically mentioned as it blocks Harry's view of Malfoy's transaction there; and the cabinet in the Room of Requirement, where it got shuffled off to after Peeves dropped it, is mentioned as a landmark when Harry is hiding his Potions book.

The question has been raised, why does Harry not get transported to Hogwarts when he hides inside the cabinet in Borgin and Burke's, in his second year? The Hogwarts cabinet is not dropped until later that year, so it should have simply transported him there. The answer is extremely simple: Harry never completely closed the door of the cabinet he was hiding in. From Harry Potter and the Chamber of Secrets: "he shot inside it and pulled the doors to, leaving a small crack to peer through." He was able to see what was going on in the shop throughout Malfoy's visit, and so the doors must have stayed open that one small crack until Harry was finally able to sneak out of the shop. If he had ever closed the doors completely, likely he would have ended up at Hogwarts.

One of the things that is stressed over and over is that Hogwarts is "unplottable", that it cannot be put on a map, and that you cannot apparate to or within it. Additionally, both in Harry Potter and the Prisoner of Azkaban and Harry Potter and the Half-Blood Prince it is mentioned that there are spells preventing entrance via any means save the gates, which in Harry Potter and the Prisoner of Azkaban are guarded by Dementors, and in Harry Potter and the Half-Blood Prince by locks that can be opened only by professors. Much of the action in Harry Potter and the Prisoner of Azkaban involves the secret ways in and out of the castle, and we as an audience are left with the impression that all such passageways are known. The Disappearing Cabinets are a way around this protection that has not as yet been guarded against.

There are two main sub-themes to the series; one, of course, is the issue of pure-blood versus muggle and the nature of prejudice. A second, less stressed theme is the idea of just consequences: Draco Malfoy tries to Jinx Harry in the Entrance Hall and is punished by being turned into a ferret; Fred and George are playing with Blood Blisterpods and get into trouble when Katie Bell gets an incurable nose bleed; Lucius Malfoy is sent to Azkaban after the battle in the Ministry and so forth. Every now and again Fred and George do something mischievous but harmful, as when they "push Montague into the old broken vanishing cabinet on the second floor" in Harry Potter and the Order of the Phoenix, and then don't get into trouble for it; however, in this case, this leads indirectly to Dumbledore's death, as it is while Montague is trapped in the vanishing cabinet that he discovers that it is connected to the one in Borgin and Burke's. Montague passes this information on to Draco Malfoy, who makes use of it as we have already seen.