A lay person is a baptized member of the people of God who has neither received the Sacrament of Holy Orders nor become a member of a religious order. A catechist is one who engages in the instruction and formation in the Catholic Faith, both for those who are preparing to be baptized and for those who are already baptized but in need of continuing instruction and formation in the Christian life. The catechist uses the catechism which is a summary or manual containing the basics of Christian doctrine. The work of the catechist is called catechesis, a term derived from Ancient Greek: κατηχισμός from kata = “down” + echein = “to sound”, literally “to sound down” (into the ears), that is, to indoctrinate. True catechesis is much more than merely instruction about Catholic Christian beliefs, values and practices. It is instruction of others plus a personal sharing of faith by committed Christians so that the entire people of God may be continually converted to a fully Christian life, that is, a life turned away from sinfulness, centered on the risen Jesus and living in hope for the everlasting reign of God.
From A CATECHETICAL PRIMER ON THE LIFE, MARTYRDOM AND GLORIFICATION OF BLESSED PEDRO CALUNGSOD by Msgr. Ildebrando Jesus Aliño Leyson