In a nutshell, and following up on the ideas in the previous question, Opus Dei is about spreading the universal call to holiness.
One of the basic responsibilities of ALL Christians is to spread the Good News. Opus Dei encourages its members and all Christians to take this responsibility seriously and provides a sort of “framework” in which to do so. The work people do in service to the Gospel message is called “apostolate.” In Opus Dei, the members’ apostolate is a “directed apostolate.” That is to say, it is supervised by the directors of Opus Dei and is discussed with the member as part of his or her spiritual direction. The director may suggest topics to bring up with specific friends, ask the member to invite someone to become a cooperator or member, suggest he or she invite someone on a retreat, etc.
Opus Dei emphasizes that members are ordinary Catholics. In terms of Canon Law (i.e., the law of the Church) this is true. Nevertheless, members of Opus Dei do have a contractual bond with the prelature that other Catholics don’t have, which obliges them to perform certain duties and give obedience to the prelature in all that relates to the prelature’s aims. See Question 5 for more details.