SPOILER ALERT: This writeup reveals some information about things that happen in Harry Potter and the Order of the Phoenix.
In Harry Potter and the Order of the Phoenix we get to visit the Ministry of Magic headquarters, and find out more about how the place, and the organization located there, is structured.
The headquarters is located entirely underground, beneath a street in a rundown part of London. The guest entrance is through a telephone box ("phone booth" for Americans), which descends into the Ministry after you dial 62442 (which spells "magic" on a phone keypad... do British phones have letter keypads?) and state your name and business. Wizards and witches more often enter the place via the Floo Network, for which there is a row of fireplaces along the entry corridor of the Atrium.
The floors are numbered downward, and their contents are as follows:
Level One: We never get to see this, but my guess is that this is where the private offices of the Minister of Magic, Cornelius Fudge, are located.
Level Two: Department of Magical Law Enforcement, including the Improper Use of Magic Office, Auror Headquarters, and Wizengamot Administration Services. Arthur Weasley works on this floor, in the Misuse of Muggle Artifacts Department.
Level Three: Department of Magical Accidents and Catastrophes, including the Accidental Magic Reversal Squad, Obliviator headquarters, and Muggle-Worthy Excuse Committee.
Level Four: Department for the Regulation and Control of Magical Creatures, incorporating Beast, Being, and Spirit Divisions, Goblin Liaison Office, and Pest Advisory Bureau.
Level Five: Department of International Magical Cooperation, incorporating the International Magical Trading Standards Body, the International Magical Office of Law, and the International Confederation of Wizards, British Seats.
Level Six: Department of Magical Transport, incorporating the Floo Network Authority, Broom Regulatory Control, Portkey Office, and Apparation Test Center.
Level Seven: Department of Magical Games and Sports, incorporating the British and Irish Quidditch League Headquarters, Official Gobstones Club, and Ludicrous Patents Office.
The Atrium: Never referred to by number, this is probably Level Eight (the "8rium"?) because it's depicted as being below Level Seven and above Level Nine. It is the main lobby of the Ministry headquarters, where the visitor and Floo entrances lead, where a security guard checks in visitors and inspects their wands, and where a bank of lifts ("elevators" for Americans) leads up and down to the rest of the place. The centerpiece of the Atrium is a group of statues of various beings of the wizarding world (a witch, a wizard, a centaur, a house elf, etc.) in politically correct harmony that in no way reflects the actual state of relations between these groups. This statue gets trashed rather badly in the climactic battle of OOtP, and I don't know if the Ministry will rebuild it. (One piece winds up in Professor Dumbledore's office when it's used as a portkey.)
Level Nine: Department of Mysteries. This plays an important role in OOtP; it would be a pretty big spoiler to reveal much more.
Level Ten: Apparently the deepest dungeon of the Ministry, this level isn't even reachable by lift; you must go down from Level Nine by stairs. Courtroom Ten is on this level; many historic trials of the wizarding world had taken place here, but it was believed to be in disuse until Harry Potter's hearing for illegally using magic outside school and in the presence of muggles was abruptly and unexpectedly moved here.