What exactly is a soul?
The glossary at the back of the U.S. version of the Catechism of the Catholic Church defines "soul" as follows:
The spiritual principle of human beings. The soul is the subject of human consciousness and freedom; soul and body together form one unique human nature. Each human soul is individual and immortal, immediately created by God. The soul does not die with the body, from which it is separated by death, and with which it will be reunited in the final resurrection.
The human person, created in the image of God, is a being at once corporeal and spiritual. The biblical account expresses this reality in symbolic language when it affirms that "then the Lord God formed man of dust from the ground, and breathed into his nostrils the breath of life; and man became a living being." Man, whole and entire, is therefore willed by God. In Sacred Scripture the term "soul" often refers to human life or the entire human person. But "soul" also refers to the innermost.aspect of man, that which is of greatest value in him, that by which he is most especially in God’s image: "Soul" signifies the spiritual principle in man. The human body shares in the dignity of "the image of God": it is a human body precisely because it is animated by a spiritual soul, and it is the whole human person that is intended to become, in the body of Christ, a temple of the Spirit. Man, though made of body and soul, is a unity. Through his very bodily condition he sums up in himself the elements of the material world. Through him they are thus brought to their highest perfection and can raise their voice in praise freely given to the Creator. For this reason man may not despise his bodily life. Rather he is obliged to regard his body as good and to hold it in honor since God has created it and will raise it up on the last day. The unity of soul and body is so profound that one has to consider the soul to be the "form" of the body: i.e., it is because of its spiritual soul that the body made of matter becomes a living, human body; spirit and matter, in man, are not two natures united, but rather their union forms a single nature. The Church teaches that every spiritual soul is created immediately by God—it is not "produced" by the parents—and also that it is immortal: It does not perish when it separates from the body at death, and it will be reunited with the body at the final Resurrection. Sometimes the soul is distinguished from the spirit: St. Paul for instance prays that God may sanctify his people "wholly," with "spirit and soul and body" kept sound and blameless at the Lord’s coming. The Church teaches that this distinction does not introduce a duality into the soul. "Spirit" signifies that from creation man is ordered to a supernatural end and that his soul can gratuitously be raised beyond all it deserves to communion with God. The spiritual tradition of the Church also emphasizes the heart, in the biblical sense of the depths of one’s being, where the person decides for or against God. (CCC 362-368)