An apparition can be an appearance of Jesus, Mary, or the saints. Apparitions are quite different from public divine revelation. Public divine revelation is about God and His plan of salvation. The last public revelation took place with the last apostle, Saint John the Beloved. After his death, public revelation ended. Subsequently only further definition or clarification made by the teaching office of the Church, known as the magisterium, took place. Public revelation is part of the sacred deposit of faith contained in Sacred Tradition and in sacred word. They are known as doctrines and dogmas, and affirmations in faith are to be our only response to them.
Apparitions belong to private revelations, and one can choose either to believe or to reject them. Even when the Church affirms a certain apparition of Jesus, Mary, or the saints as being authentic, one does not have to accept it. When the church declares an apparition as being authentic, it does so in order to differentiate from false prophesy and, consequently, to aid Christians on their journeys of faith.
Apparitions are different from locutions. Apparitions are images imprinted on the senses of the visionary; locutions are speeches received from the visionaries. Not everyone can see or hear apparitions or locutions. In most of the Marian apparitions (apparitions in which Mary appears), there is a message of the Blessed Virgin to pray to her Son and to do penance. Apparitions also take place over the course of the centuries to encourage the faithful to remain close to her Son and to the Church. Sometimes this is achieved by commencing a new devotion.
For example, Our Lady (an affectionate way Catholics refer to the Virgin Mary) appeared to Saint Catherine Laboure, a nineteenth-century sister of charity in Paris. She instructed this nun to create a new medal, the miraculous medal with the words, “O Mary conceived without sin.” In Fatima, Our Lady appeared to three little children—Jacinta, Francesco, and Lucia. She asked for total consecration to her Immaculate Heart, for prayer for conversion of sinners to avert war, and for an end to communism. The devotion that began was the five first Saturdays in honor of Mary Immaculate.
It is important to note that there are relatively few apparitions or locutions that are authentic. (Many who have made claims are either misguided at best or deceivers at worst.) The Church declares an apparition only after conducting a rigorous investigation. Ultimately, though, all we need to know in order to attain heaven has already been publicly revealed to the apostles and further defined by the Church.