Recent developments in Rome and Canterbury and the scandalous pro-abortion activities of a Dominican sister have raised a number of questions about the ordained ministry of the Church and religious life. There's a lot of confusion out there!
Let's see if we can bring some clarity to the scene. Before moving on, I have to point out that almost every distinction and claim made in what follows has its exceptions. I am making no attempt here to be absolutely thorough. These are general distinctions made in order to clarify terms for average Catholic folks. Objections can (and no doubt will) be made to just about everything I've written here.
1). What is the difference between a sister and a nun?
All religious women are properly called "Sister." Some religious women are nuns and others are sisters. Nuns are solemnly professed religious women who live contemplative lives within the confines of a monastery. Nuns are cloistered sisters. Sisters (or "apostolic sisters") are solemnly professed religious women who live "in the world," working at various jobs either directly or indirectly associated with the Church. Some sisters live in convents. Others live alone or in small groups in apartments or houses. Nuns typically wear some kind of habit. Sisters rarely do.
2). Monastery vs. convent?
Monasteries are for men, convents for women, right? Wrong. These two terms denote whether or not the religious men or women are cloistered, that is, enclosed. Religious who live enclosed lives live in monasteries. Religious who live apostolic lives live in convents. Monks and nuns live in monasteries. Sisters and friars live in convents.
3). Friar vs. monk?
Friars are a relatively new form of religious life (13th c. or so). We are partly cloistered and partly apostolic. Friars are free to leave the cloister when the occasion requires it. Monks are cloistered all the time and leave only with permission. For both friars and monks, modernization has loosen regs on leaving the cloister. Another distinction that I'll throw in here is the difference between a "Father" and a "Brother." Most male religious orders have members who are ordained priests and members who are laymen. Dominicans have clerical friars and cooperator brothers (i.e. "lay brothers"). Historically, lay brothers in the Order were the laborers among the clerics. They kept the practical side of priory life going while the priests contemplated the mysteries of philosophy and theology. Brothers basically served the community in the kitchens, the garages, the yards, the laundries. With the advent of Vatican Two, the decline in vocations generally, and the rise of egalitarianism in the Church, the brothers stepped up and took on ministries normally reserved to priests. There was some significant pressure in the 1980's in response to the priest shortage for brothers to be ordained. Many did so. Nowadays, cooperator brothers earn PhD's in just about any field useful to the Order, serve in parishes, universities, chancery offices, etc. Unfortunately, in many Dominican provinces, the lay brother vocation is on the decline, if not altogether extinct. Currently, there is a move to reinvigorate the vocation. Deo gratis!
4). Secular priest vs. religious priest?
All Catholic priests are either secular or religious. This is not a distinction between degrees of holiness or religious observance. Secular priests work directly under a diocesan bishop. They make promises of celibacy and obedience. Typically, secular priests work in parishes or diocesan schools. They can be assigned outside their diocese by their bishop. Secular priests wear black clerical suits when "on duty." Religious priests are ordained men who belong to one of the Church's many orders, societies, or congregations, e.g. Benedictines, Jesuits, Dominicans. Religious priests typically follow a rule of life designed by a saint or some other spiritual leader. Religious priests take vows of celibacy, poverty, and obedience. Religious priests wear distinctive habits, or sometimes black clerical suits. Secular priests tend to live alone, while religious priests tend to live in community. This is not a hard rule, but it is almost always the case.
5). Vow of poverty?
Different orders have different philosophies of poverty. For example, Franciscans typically see poverty as an end in itself. To be poor is the goal, a sign of dedication and holiness. Dominicans typically see poverty as a means to improving the preaching by eliminating distraction and material commitments. Generally speaking, the vow of poverty commits the religious person to living within the means of his/her community for the benefit of the community. Salaries, bonuses, donations all go into one communal pot and individual expenses are paid out of the community's resources. For example, as a friar I do not own a car. When I am assigned to a priory, the prior of the community assigns me a car to use while I am a member of that community. Gas, insurance, maintenance are all paid out of the priory's communal pot. For Dominicans, poverty does not mean impoverishment but rather something closer to simplicity in community life. For Franciscans, it means something very close to impoverishment. Theologians and philosophers are still trying to understand the notion of "Jesuit poverty." ;-)
6). Vow of celibacy/chastity?
Three terms need to be defined here. Celibacy means not getting married. Chastity means being faithful to your commitments. Continence means no sex. Reporters in the secular media often confuse these three terms. Vowed religious are bound to chaste celibate continence. Celibacy and continence might be thought of as restricted behaviors: no marriage, no sex. Chastity is an attitude. True chastity is extraordinarily difficult to achieve. Married people are called to chastity. So are single people. Since Catholic moral theology teaches that sex activity is only morally permissible within a sacramental marriage, celibacy always means continence. So, if a Catholic is single, he/she is also called to continence.
7). Vow of obedience?
Again, lots of philosophical differences among the various orders of religious. Historically, Jesuits have used a military model for obedience. Get your orders, follow them. Period. Dominicans used to be somewhat like this, but now we tend to see obedience as something more akin to paying careful attention to the needs of the Order and responding to those needs with generosity and good will when asked to do so. It is rarely the case that a Dominican must be "ordered" to do something. However, it can be done with a "formal precept." Most assignments in the Order are made after careful consideration for the gifts and temperament of the friar. No provincial wants to assign a friar to a ministry that he is loathe to take on. This means unhappiness all around. Abuses of authority in the past have led many religious orders to abandon the military model of obedience. Now, we tend to focus on the root meaning of obedience and emphasize the necessity of "listening" to the needs of the community and responding in generosity. This too causes problems, but so long as we are on this side of heaven we must deal with human fallenness. Obedience is meant to mitigate our natural tendencies to seek out our own good regardless of costs.
8). Habits: yea or nay?
This is a minefield. If you want to start a shouting match among religious, pronounce on the issue of habit wearing. Regardless of how you come down on the issue, someone will object and they will usually object loudly. Within communities, habit wearing (or not) has become a symbol for all the ideological fights that we are fighting in more detail in other arenas. For example, habit wearers are traditionalists, conservative, authoritarian, medieval, and seeking after attention and privilege. Non-habit wearers are reformers, liberals, loosy-goosy hippies, modernists, and trying too hard to be hip. Wear a habit and you proclaim your ideological allegiance to institutional conformity and power. Don't wear a habit and you proclaim your ideological allegiance to non-conformity and libertinism. Of course, these are caricatures. Most religious wear habits when appropriate. The real fights begin when either side tries to impose its habit wearing views on the other. Typically, there are times and places where habits are specifically called for, e.g. communal prayer, meals, ministry. And there are times when the habit wearing is discouraged, e.g. casual shopping, outside work/recreation, etc. And, as always, there are exceptions to these rules! Local communities usually have their customs about what is and what is not an appropriate time and place to don the habit. Younger religious these days tend to wear the habit more often then not. Interestingly, habit wearing isn't a big issue among European Dominicans. And they seem to think that American Dominicans are being silly to fight about it. Welcome to religious life!
What have I left out?