From Ars, 3rd Edition, 4th Edition (more or less):

All Hermetic Magi belong to the Order of Hermes. This order was founded in 767 by the great Magus and philosopher Bonisagus. Bonisagus codified the laws of magic which allow all possible magical spells to be created using the ten Forms and five Techniques. It is his discovery which made the sharing of magic, and thus the Order, possible.

Bonisagus created the Order of Hermes out of the remains of the Roman cult of Mercury, a magical organization in Classical times. After the collapse of Rome and the cult of Mercury, based as it was on ritual magics requiring hundreds of participants, Bonisagus worked with the Maga Trianoma to unify all of the Magi of Europe. A handful of Magi, from both Celtic and Classical backgrounds, joined with Bonisagus in becoming the founders of the Order of Hermes. They were then joined by Bjornaer, a Maga from a Germanic tradition.

These twelve Magi formed the twelve Houses of the Order, each named after its founder: House Bjornaer, House Bonisagus, House Criamon, House diedne, House Flambeau, House Jerbiton, House Mercere, House Merinitia, House Quaesitoris, House Tremere, House Tytalus, and House Verditius.

In the early Ninth Century, the Maga Pralix, a student of Tytalus, founded the thirteenth and final House, Ex Miscellanea, which exists primarily to bring non-Hermetically trained Magi into the Order. In the early Eleventh Century war broke out in the Order, and all members of House Diedne were killed or driven into hiding, and the House was disbanded. There are now twelve Houses in the Order.

Houses of Hermes:

House Bonisagus:

Bonisagus invented the Hermetic theory of magic, and his student Trianoma masterminded the formation of the Order, so this House has always enjoyed a great deal of respect from other Magi. Bonisagus Magi continue both the theoretical and political traditions, some refining and expanding Hermetic theory, others overseeing the continued political development of the Order.

House Tytalus:

In their never ending search for conflict, the leaders of House Tytalus went too far in the Tenth Century, and were corrupted by demons. In their arrogance they believed they could master the dark forces, but even those masters of intrigue could not outwit the Hell-spawn. They were executed for the crime of Diabolism, and House Tytalus has been distrusted and in disgrace ever since.

House Jerbiton:

House Jerbiton is interested in the mundane world and assumes the duty of keeping the Order on good terms with the Church and nobility.

House Criamon:

This highly secretive House is known for its obscure philosophy, its disdain for simple power, and the habit of its members for marking their faces and bodies with arcane symbols.

Those of this House seek the experience of the "Enigma," which seems to be some sort of mystical experience.

House Bjornaer:

These wizards have been only partially accepted into the Order because their founder was from a Germanic rather than Roman tradition. Their fascination with beasts and the animal side of human nature makes many Magi uneasy. Bjornaer Magi value and appreciate the animal side of their true nature, as well as the human. All Bjornaer Magi learn to take the shape of an animal at will, called a "Heart-Beast," which in some way reflects the inner self of the Magus.

House Verditius:

The Magi of this House depend on magical devices in order to cast their magic. Verditius Magi have achieved significant power by making themselves invaluable to other Magi, creating enchanted items for others.

House Flambeau:

These aggressive and ferocious Magi often cause trouble in the Order and are known for getting mundanes angry at the Order on a regular basis. Their fearlessness and love for destructive spells make them invaluable on Wizards' Marches and other major military operations. Those of this House believe that the world is full of people and things who cause trouble, and that the only solution is to burn them. While most of them specialize in Ignem, some Flambeau Magi have taken up Perdo as a more "subtle" alternative.

House Merinitia:

This House is focused on the world of faeries, and its members tend to be just as strange as the creatures they study. Those of this House eschew the merely mortal, and seek the world of the faerie.

House Guernicus:

This house is usually referred to as House Quaesitoris. Members of this house are the judges of the Order, investigating wrongdoing and passing sentence on those who transgress the Code of Hermes and the Peripheral Code. They believe the Order will collapse through internal conflict without their fierce stewardship.

Although House Guernicus trains and inducts their own apprentices, other magi can also join their ranks. One of the highest honors in the Order is to be invited by the elders of House Guernicus to become a Quaesitor. Such magi usually retain membership in their original House.

House Ex Miscellanea

This is a large, diverse, and highly disorganized collection of magi. Though originally founded by a renegade as a rival to the Order of Hermes, this association was eventually accepted as a house of its own. It accepts wizards of all kinds, many only nominally Hermetic, whose magic comes from many disparate traditions. Magi Ex Miscellanea are often called hedge wizards by their many detractors within the Order, and in fact, most of its membership originated in the peasant class.

House Tremere

Members of House Tremere emphasize the importance of judgment, strategy, and detailed planning. They believe in the respect of superiors and in asserting authority over minions. Dignity is of the utmost concern.

(While Ars was a holding of White Wolf, House Tremere was a lead in for the Tremere vampires. This never happened. They aren't connected to vampires.)

House Mercere

The founder of this house lost his magical powers but remained involved in the Order. He assumed a non-magical role valuable to other magi - that of messenger. His followers continue to fulfill that role. All members of House Mercere, regardless of whether they possess the Gift, are officially recognized as magi of the Order. Even non-Gifted Mercere spend 15 years in apprenticeship like other magi.

Followers of Mercere are more commonly known as Redcaps because of the headgear they wear as a badge of office. Redcaps are permitted to attend and vote at tribunal, and are under the protection of the Hermetic Code.

The Hermetic Oath:

I, name, hereby swear my everlasting loyalty to the Order of Hermes and its members.

I will not deprive nor attempt to deprive any member of the Order of his magical power. I will not slay nor attempt to slay any member of the Order, except in justly executed and formally declared Wizards' War. I hereby understand that Wizards' War is an open conflict between two magi who many slay each other without breaking this oath, and that should I be slain in a Wizards' War, no retribution shall fall on he who slays me.

I will abide by the decisions made by fair vote at tribunal. I will have one vote at tribunal, and I will use it prudently. I will respect as equal the votes of all others at tribunal.

I will not endanger the Order through my actions. Nor will I interfere with the affairs of mundanes and thereby bring ruin on my sodales. I will not deal with devils, lest I imperil my soul and the souls of my sodales as well. I will not molest the faeries, lest their vengeance catch my sodales also.

I will not use magic to scry upon members of the Order of Hermes, nor shall I use it to peer into their affairs.

I will train apprentices who will swear to this Code, and should any of them turn against the Order and my sodales, I shall be the first to strike them down and bring them to justice. No apprentice of mine shall be called a magus until he first swears to uphold this Code.

At this point, followers of Bonisagus recite: I shall further the knowledge of the Order and share with its members all that I find in my search for wisdom and power."
All others recite: I concede to Bonisagus the right to take my apprentice if he should find my apprentice valuable to him in his studies.

I request that should I break this oath, I be cast out of the Order. If I am cast out the Order, I ask my sodales to find me and slay me, that my life not continue in degredation and infamy.

The enemies of the Order are my enemies. The friends of the Order are my friends. The allies of the Order are my allies. Let us work together as one and grow hale and strong.

This oath I hereby swear on date. Woe to they who try to tempt me to break this oath, and woe to me if I succumb to the temptation.

--Alexei Tymoczko