Opus Dei is neither a religious order, like the Dominicans, Franciscans & Jesuits, nor is it a Secular Institute or religious movement, like Cursillo or Charismatic. It is a Personal Prelature, 95% which is laity and only 5% clergy. It was founded by the late and recently beatified Blessed Jose Maria Escriva in the early thirties in Spain. Well before Vatican II taught the UNIVERSAL CALL TO HOLINESS of all the baptized, Msgr. Escriva sought to promote a spirituality for the laity. Previously, the only spirituality was that of the religious monks and nuns. The monastic life, especially the rule of St. Augustine and St. Benedict was the keystone of spirituality. The mendicant orders, Dominican & Franciscan, were a modification of these. The parish priest, i.e., the Diocesan (Secular) Priesthood was a further modification of those. Hence, all Priests, regular (religious) or secular (diocesan), must pray the Divine Office. The Liturgy of the Hours is designed in a monastic style, with the hours of the day broken up as the monks divided their day.
The laity had to adapt and dilute the already attenuated monastic spirituality for themselves. Consequently, laypeople had no spirituality which was uniquely their own. It was nothing more than a watered down version of a religious spirituality. Msgr. Escriva found this unacceptable, save for those laity who felt called to embrace the Third Order (tertiaries) of Dominicans, Franciscans, Carmelites, et al. He devised a spirituality of and for the laity. OPUS DEI, the Work of God, is a means by which the Catholic Faithful sanctify themselves and the world in which they live and work. It is comprised of all walks of life, doctors, lawyers, homemakers, teachers, students, bus drivers, retirees, etc. The goal is for each member of Opus Dei to bring their Roman Catholic Faith into their WHOLE life, home, work & play. As leaven in the world, the laity being IN the world bring Christ and the Catholic Faith INTO that same world by the way they practice their Faith. The clergy's function is to HELP the laity find their spirituality and to help them bring the FAITH to the world.
Opus Dei, then, is a vehicle by which its members sanctify the world by sanctifying themselves in whatever situation and condition and vocation they find themselves. The so-called "secret" of Opus Dei is that the members do not wear their religion on their armsleeves. They are very well read in the Magisterial teachings of the Church, are very loyal to the Holy See, and they quietly but effectively defend Church dogma and faithfully practice their Catholic Faith 24 hours a day without crediting it to Opus Dei, necessarily. Their absence of self-publicity breeds contempt from their enemies who see them as clandestine. In reality, it is nothing more than humility. Opus Dei members study the Faith and they INFUSE Catholic virtues into a secular world and secular society. Rather than selling out to the social mores of the pagan culture like modern sycophants, Opus Dei members uphold the moral and doctrinal teachings of the Church and encourage all men and women of all faiths to obey the Natural Moral Law. Due to their resistance of diluting Church law, many opponents accuse Opus Dei of being anti-ecumenical.
All in all, Opus Dei is a superb method for any Catholic Christian to know their Faith more fully, to create a concrete strategy for Christianizing the world and to build up the kingdom of God via ALL members of the Church, lay and clergy. As it is more cerebral and intellectual, it may not attract everyone just as the Cursillo and Charismatic Movements are not for everyone either. It is fully legitimate and fully recognized by the Church, from the Roman Pontiff on down. All I can say is to check it out by attending a day of recollection, make a retreat or talk to some members of Opus Dei to see if it is for YOU. Read Msgr. Escriva's monumental work, "THE WAY" and see if it helps you in developing a spirituality; a plan of life. If not, at least it won't be a waste of time by any means.