There are three types of members in the men’s branch: numeraries, associates, and supernumeraries. The vocation is the same for all 3 types. The distinction is in their availabilty to direct and assist in the apostolic activities of the Prelature.
Numeraries are the most available. They live celibacy and give all their free time to the Work. They are responsible for their own financial support, and after meeting all their personal financial obligations, they give whatever money is left over to the Work for use in its apostolates. As a general rule, they live in centers of the Work. They receive an intense formation in the philosophy and theology of the Church. Most of them hold regular secular jobs, but for some of them their professional work is to direct the apostolic activities of Opus Dei or to hold an internal position in the governance of the Prelature. For most of those who hold internal positions, this is a temporary situation. The numeraries are the primary givers of spiritual direction to the rest of the membership, and the intense formation they receive prepares them for this role. They are at the disposal of the Prelature and are generally ready to move wherever the Prelature needs them.
It is generally from the numeraries that the prelate calls men to the priesthood. When a man becomes a numerary, he does so with the willingness to seriously consider the possibility of a vocation to the priesthood if the prelate should ever ask him. However he may decline the invitation. A very important point is that he does not become a numerary with the intention of becoming a priest. Rather, he simply remains open to seriously considering the possibility if the prelate asks him.
When you hear horror stories about life in Opus Dei it is generally in reference to life as a numerary. See Question 49 for a discussion of what I believe is the source of these stories.
Associates are the next type of member, in order of availability. Associates are similar to numeraries, in that they live celibacy, but they typically do not live in centers of the Work. Their personal circumstances do not permit them to be as available to Opus Dei as a numerary. Perhaps they have an elderly parent they have to take care of, or they run a family business that would interfere with their ability to move to another city. There are a whole host of reasons they would be less available than a numerary. Associates also are involved in giving spiritual direction to other members of the prelature and to non-members, too. The prelate may also ask associates to become priests. They also remain free to say no.
Supernumeraries are the third type of member. These are the least available to Opus Dei. Supernumeraries may be married or unmarried. They live wherever they want. Most of the members are supernumeraries, and it is they who carry out the real apostolate in Opus Dei. It is in the supernumeraries that “the rubber hits the road,” so to speak, because, as St. Josemaría used to say, the real apostolate of Opus Dei isn’t the schools, the hospitals, etc. The real apostolate of Opus Dei is the apostolate carried out by its members among their colleagues, their friends, and their families.
Both the women’s branch and the men’s branch have numeraries, associates and supernumeraries, and they perform the same functions in each branch. While the women numeraries can’t be ordained, they receive the same philosophical and theological formation the male numeraries receive.
There is another type of member in the women’s branch called “numerary assistant.” Numerary assistants attend to the domestic needs of the centers of Opus Dei, both for the men and for the women. They run Opus Dei’s conference centers. They do all the cooking and cleaning. They take care of the chapels (actually, they’re called oratories, but that’s addressed in Question 9). St. Josemaría used to call the work they do the “apostolate of apostolates” because it is they who imbue the centers with the family spirit that characterizes Opus Dei, and their work frees up the other members to devote more time to giving formation.