In the Church, we typically refer to a vocation as a permanent state of life, a call from God to an individual person to consider marriage, priesthood, or the consecrated life (i.e., brothers/monks/friars & sisters/nuns). Sometimes the words vocation and calling are also used to refer to more general activities outside of marriage, priesthood, or consecrated life, like a vocation to be a teacher or a calling to do service work for a year. In fact, all baptized Christians have a universal call to holiness, to live in a manner befitting a child of God, and to sanctify the world wherever we live. But a vocation, in the formal sense, is oriented at living out a particular lifestyle that transcends above a career choice, a temporary phase of personal growth, or the universal call to holiness. It is a permanent state of life, initiated by God, responded to by those who discern the call through extensive prayer and formation, and officially (canonically) recognized by the Church.
All of us are naturally made for marriage, insofar as our bodies are fit for procreation and our hearts are oriented to form a communion-relationship with another. Most every Catholic is called to marriage, and therefore we ought to spend a significant part of our life as young adults preparing for this vocation. It is meant to be a partnership for the whole of life, for the good of the spouses, and for the good of the generation and education of children. None of those three things can be left out, otherwise it is not really marriage. Of course, most young people think about dating, romance, and the sexual relationship formed between a man and a woman. Still, love is not a reckless endeavor without boundaries. We all have feelings, needs, and wants that we bring into a relationship. This is why having good boundaries of behavior, positive character traits, a healthy sense of self-donation and sacrifice, prudence, and wisdom are important to preparing to enter a partnership for the whole of life. Romantic love is not enough to hold a marriage together. It takes a love that is rooted in friendship with God, since God is love.
Some men are asked by the Lord to give up the vocation of marriage and be ordained as a celibate (non-married) priest, all for the sake of the Kingdom of God. Priests are at the service of those who are married and single, the Body of Christ. Priests are given the responsibility to build up the souls of the faithful, especially through the sacraments and prayer. Some look at the sacrifice of marriage as too much for them, but the Lord provides the graces needed to do it, much in the same way a married man is offered grace to remain faithful to his wife through chastity and sexual continence. The priest seeks to conform his life to that of Jesus, whose entire life was focused on doing the Father's will. He freely offered himself as spouse to the Church, and so too, the priest espouses himself to the Church.
Consecrated (Religious) Life
The Lord calls some men and women to live a consecrated celibate life according to the evangelical councils, so as to remind the Church of the universal call to holiness. The Lord gives to them a special charism, a gift of the Holy Spirit to be shared with the Body of Christ. A charism is a sort of highlight about who Jesus is, lived out by those to whom it is given.
E.g., St. Francis of Assisi was given a charism of poverty, of which he desired to be poor and serve the poor, just as Jesus did. Mother Theresa of Calcutta was given a charism to be a missionary of charity. St. Dominic was given a charism to preach and teach. St. Thérèse of Lesieux was given a charism to show love in little ways. St. Ignatius of Loyola was given a charism to educate souls through spiritual exercises.
Typically, those called to the consecrated life have a desire to live in community, to live a chaste life for Christ, and to perform some kind of service to the Church, whether it is an active ministry, like health care, or a more passive ministry, like spending the day in intercessory prayer. Likewise, some communities have multiple houses from where they work, and some have only one house where they live, work and pray. Those who live in cloister usually do not have interaction with the public except in rare circumstances. Communities of men or women who live at only one house are usually referred to as monks and nuns. Those who live in multiple houses are usually referred to as brothers and sisters, and some men are referred to as friars. The title "brother" or "sister" is given to all of the above, even if they are a monk or nun, friar or otherwise.
All consecrated persons live according to the evangelical counsels of poverty, chastity, and obedience. Some make solemn vows to live them permanently. Others make simple promises. Some live in together in community, and some live them apart from a community. The differences depend on the nature and character of the charism given to the founder of the group.
Discernment of God's Call
It takes time and lots of prayer to clearly understand God's subtle movements of grace. Every once in a while the Lord is direct and explicit in how he lets us know what we should choose. However, most of the time the Lord wants us to listen carefully over a period of time, to discern what God is saying, not just jump at the first intuition or fear we have. We should pray often, seek the wisdom of those who have done good discernment before us, and learn what graces God is giving in our personal situation.
A more in-depth overview of discernment by Fr. Aaron Kuhn
Fourteen steps to discern a vocation by Fr. Gregory Mastey
A Prayer to Discern God's Call
Heavenly Father, you formed me in my mother's womb. In baptism you adopted me as your son/daughter. You know everything about me, especially what will make me happiest and how I can serve you best during my short time on Earth. By Your Holy Spirit, grant to me whatever graces I need to hear your voice and follow your Son, Jesus, the Good Shepherd. I abandon my own plans to you, so that Your will may be done in and through me. I ask this prayer through Christ, our Lord. Amen.