OSR: Dear Abbey, my sister has this habit...

(For those of you who don't get the reference.)

In my games, being a member of an Estate is important. Anyone* can start the game as a member of the First Estate. Being a priest or a monk or a nun isn't a class, really**. It's just a thing you can do on top of being something else. It doesn't give you any special skills or options. It's more like picking a political affiliation or career advancement track. This post covers some details about being a monk or a nun. To learn how to think like one, read these posts.

Not very useful specialized terms (can be ignored)
Cathedrals were served by canons secular, who were sort of like monks, but not quite.

Convents housed mendicant nuns but the term was also used for a house of friars, except...

Friaries house friars.

Monasteries house monks.

Nunneries house monastic nuns.

Priories were small satellite communities attached to a main monastery.

Cloisters are just another term for this type of community in general.

It's all very complicated and highly variable and, from a gaming point of view, not very useful. Let's move on at top speed.

Useful Terms

Abbey

A religious community, isolated from the rest of the world. Usually walled, often built on a regular plan, and controlled by a set of rules. I'm going to use "monastery" and "monastic" interchangeably with "abbey", mostly because there's no good adjective form of "abbey".

Abbot/Abbess

The leader of this religious community. Sometimes equivalent in rank and prestige to a Bishop, but not an ordained member of the Church. Also has limited religious authority, and usually defers to the local Bishop in most matters. They are elected by members of the monastery rather than being appointed from above. Some abbots were ordained and could perform services, but most of the time, the community relied on the local priest.

Monk/Nun
A respected member of a monastic community. There might be dozens of degrees of monk-ness within one order. We're not going to cover them. Monks and nuns refer to each other, and are referred to, as "Brother" or "Sister", to emphasize the nature of the community and to deliberately show their lack of concern for familial bonds or the vanity of rank.

Initiate
A provisional member of a monastic community. Usually there's a waiting and training period of several years between entering the community and becoming a monk or a nun. Initiates have different tasks and fewer restrictions.

Holy Orders
Rather than the near-feudal obligation a parish priest owes his bishop, members of a monastic community instead take solemn vows to the divine presence alone. The vows almost always included celibacy, isolation and immobility, a renunciation of worldly things (wealth, vanity, comfort, etc.), and a strict duty to obey. Over and over monastic rules stress the duty to obey both God, the Church, and other monks. Monastic saints tales always stress their obedience (especially in Russia, where obedience to the duties of monastic life is contrasted with disobedience to duties to your parents). This obedience could sometimes go to supernatural lengths. 

I was told a story of Father Bede Jarrett, the great Dominican provincial who died not very long ago, which illustrates that. I have been told that when he fell into his last illness, Father Bernard Delany went to see him, and said, "Well, Father, of course you know that you've got to get well; we can't possibly spare you". And about a fortnight later, when Father Bernard went to see him again, Father Bede said, "Oh, Father, I'm so dreadfully tired; do you think you could let me want to die after all, or must I go on under obedience wanting to live?" And he naturally said, "Oh, of course I never meant to put you under obedience". And Father Bede said, "Thank you so much", and died about half an hour afterwards. 

-The Creed in Slow Motion, Ronald Knox

Yet it was an obedience without authority. You didn't obey your peers because of their rank. You obeyed them because it was a joy to obey. The usual order of the feudal world was inverted; to seek authority was a sin, and to glorify in your power was a very dangerous sin indeed. Most tales of saintly abbots and abbesses have them reject the choice of their peers several times. Abbots who were sent away to become bishops often fled back in tears.

Holy orders also stressed humility, poverty, and simplicity. Some orders required manual labour, others focused solely on the spiritual labour of prayer. Everyone had a daily routine.

Tonsure

The distinctive haircut of a monk, meant to be deliberately weird-looking and unfashionable.

Habit

The distinctive clothing of a member of a monastic order. Varied widely between orders and centuries. For men, usually a black or brown robe with a hood. The hood is not always worn. For women, a black and white robe with a hood or head-covering, sometimes quite elaborate, which is almost always worn. All the different parts have specialized names. It's not relevant here.

*Except Knights and Wizards and obviously unsuitable people.

**I've written an official Monk class if you want to play a character who is only monastic. The First Estate is open to most characters.

Initiate Options for Level 1 Characters

If you want to start as a monk or a nun, start as a monk or a nun, adapting the background from your class as needed. Here are a few ideas.

Fighter: A former soldier, seeking to escape their past. A persecuted mercenary, hiding from their enemies. A secret order, devoted to fighting a mortal threat. A bloodthirsty warrior-monk.

Thief: A spy for the Church. A penitent trying to mend their ways. A dissolute noble, forcibly imprisoned. A sanctioned relic-stealer (if you can steal a relic, that means the saint wanted it stolen).

Barbarian: A distant monk from Foreign Parts, sent to instruct the locals. Someone fleeing persecution. A new convert, fanatical and naive.

Summoner: A secret cultist with a holy facade. A holy order, kept secret and deployed for emergencies. Someone who read the wrong kind of book.

Paladins: N/A. With a direct connection to the Authority, Paladins are Outcasts; the structure of medieval society is like a stage set made from twigs and wax paper.

Knights: N/A, part of the Second Estate. 

Wizards: N/A, being a Wizard prevents you from taking Holy Orders.

Cats. Assholes then, assholes now.

The Narrow Path 
Aside from the spiritual labour of prayer, monks and nuns often worked to copy manuscripts, write new commentaries, maintain hospitals (which were both places of healing and houses of rest for travelers and the poor), grow herbs, translate manuscripts, advise kings, and generally "do good works." Some orders were required to maintain schools. Education in the middle ages was dominated by the monastic orders, and their ideas, drawn from ancient books of law, lore, and poetry, informed countless generations. Many monks and nuns were a comfort to the sick and the dying, counselors in disordered times, sharp critics of the powerful, and true to their vows.

There was also some joy in monastic life. Monks and nuns belonged to community with minimal opportunity for strife and trouble compared to the outside world. An abbey could also serve as a shelter for the persecuted, the widowed, the orphaned, or people simply unsuited for life elsewhere. 

The Sin, Heinrich Lossow.
 The image isn't NSFW. That helpful man is just assisting a poor nun stuck in a gate. Shame on you.

The Other Path
Monastic orders follow a very predictable arc. They are founded by a visionary leader disgusted by other monastic orders, the Church, or the world in general. The visionary acquires a small group of followers, all of whom live in peace and equality. Then more people arrive, and some kind of structure is needed. The visionary introduces one and then dies. The successors try to keep the tradition going.

By this point, the order's fame has spread. Nobles donate vast sums, because the order is so humble and pure and therefore much better than  the other, older, and more corrupt orders. The Church interferes and tweaks the less dogmatically sound rules. The order expands, creates new branches, invents new rules, acquires a few less scrupulous followers, and gradually becomes no better or worse than the other orders. The appearance of new orders alone would be testament to the less-than-devout state of many monasteries. Peasants and Popes constantly complained of drunken, lecherous, ignorant, and idle monks - idleness being the main concern. If the monks weren't performing spiritual labour, they weren't much use. Abbots became as proud as princes. If the First Estate, the moral guides of the other Estates, failed to act properly, of what use was the entire system?|

Monastic life was not easy, and not all initiates were willing. Unwanted children, marriageable daughters, wilful, spiteful, or mad heirs, or potentially dangerous claimants were shipped off to abbeys wholesale. They could not leave; breaking a sacred vow meant excommunication and pursuit by both Church and State. Under the circumstances it is hardly surprising that many men and women rebelled against vows they did not choose to take, and a life whose monotony, isolation, and discomfort could crush the soul rather than purify it.

The whole monastic ideal is, however, bound up with the vow of chastity, and had only women with a vocation entered the nunneries, the danger of the situation would have been small. Unfortunately, a large number of the girls who became nuns had no vocation at all. They were given over to the life by their family, sometimes from childhood, because it was a reputable career for daughters who could not be dowered for marriage in a manner befitting their estate. They were often totally unsuited for it, by the weakness of their religious as well as the strength of their sexual  impulses.
[...]
The ancient house of Amesbury fell into evil ways in the 12th century. In 1177 its abbess was said to have borne three children and its nuns were notorious for their evil lives, whereupon the convent was dissolved... Later still the papal benevolence was exerted on behalf of Margaret Greenfield, nun of Amesbury, who had borne a child after her profession (1398) and Cecil Marmyll, who "after having lived laudably for some time in the said monastery, allowed herself to be carnally known by two secular priests and had offspring with each of them." (1424).

-Nuns and Nunneries: Sketches Compiled Entirely from Romish Authorities, Lewis Tonna

We don't know much about the specific details of many alleged offenses because the Church, to protect its own interests, always fought to hide any scandals.

Structure of an Abbey

This article covers the basics. An abbey is a simple, enclosed structure. Most had outbuildings, farms, and secondary structures, but the core of the complex was a closed and walled community. Draw a square and stick a church on one side.

Abbeys could sometimes serve as minor fortresses and shelters for local villages during times of war, but they were rarely designed as defensive structures. They relied on isolation, the power of the Church, and the protection of local feudal overlords and sponsors. In disordered times, many monasteries were plundered by enemies and allies alike. They were also very vulnerable to raids. An isolated location meant protection from passing armies, but small forces from the sea, rivers, or overland could easily surprise and ransack an essentially defenseless community.

The elaborate Mont Saint-Michel

The imposing and ancient Sacra di San Michel

Further Reading

This website covers everything I've left out, ignored, or simplified. It's well worth a look.