A cleric wears the dress prescribed by Canon Law and other ceremonial directives as a mark of his humility and obedience to the Church, his death to the world, and his complete embracing of his sacred vocation. Indeed, clerics are essentially supposed to be in some form of clerical dress at all times, with exceptions like participating in athletics. So, let's look at what the clergy wears and why they wear it.
This is the fundamental clerical garment. It symbolizes the cleric's death to the world and shedding of worldly vanity as he humbly submits in full obedience to the Church and service to God.
The cassock is actually a long, ankle-length coat, not a robe. It is worn by all clerics and even seminarians. For most, it is black. For Bishops and certain other Prelates, it is purple as a sign of their participating in the fullness of Christ's Holy priesthood as the principal ministers of the Gospel. For Cardinals, the cassock is red as a sign of their willingness to defend the faith even to the point of spilling their own blood. The Holy Father's cassock is white, a custom that began when a Dominican, Pius V, became Pope and retained his white Dominican habit. For those with colored cassocks, there is also a "house cassock" in black with red or purple trim for use in non-liturgical settings.
The cassock is always correct attire for a cleric. For black and white tie functions, the cassock is what Catholic clerics are generally expected to wear, along with a cape called a ferraiolo (see below). Sometimes a formal clergy suit is permitted. Most formally, it is worn with knee britches and stockings. This custom is stil seen today, especially in England. For Anglican Bishops, there is a special variation called apron and gaiters, which has a jacket in black or purple similar to a cassock, but short, and black gaiters are worn over the stockings. This originated as a practical garment for wear on a horse as the Bishop travelled and symbolizes the Bishop's duty to visit his flock.
Over the cassock is worn by major clerics and some seminarians the fascia, a wide band of cloth with usually fringed ends that hand down on the left side. It is black for most clerics. Bishops and prelates use a purple one. Cardinals use red, and the Holy Father uses white. Some seminaries have special colors used for the fascia and even the cassock. The fascia symbolizes chastity according to one's state in life.
The ferraiolo is a full-length formal cape attached at the neck with a ribbon tied in a bow. It is worn over the cassock by priests, especially for formal occasions, and demonstrates the glory and grandeur not of the individual, but instead of Christ's priesthood. It is black for most clergy, purple for Bishops, and red for Cardinals. The Pope does not use a ferraiolo.
Proper to all clerics, the zucchetto is a skull cap that represents the clerical tonsure. It may be worn by all who have received First Tonsure and continues the symbolism of the cleric as a slave to Christ and humble servant of the Church. It is usually black for all clerics, though those in religious orders may have zucchetti in colors proper to their order. That for Bishops is purple, for Cardinals red, and for the Pope, white. Some prelates have black zucchetti with purple or red trimming.
The saturno is the standard non-liturgical clerical hat. It is primarily a practical item for wear outdoors with the cassock and sometimes with the clergy suit. It is usually black and may, for priests, optionally have black cords and tassels. For Bishops, green and gold cords may be used, and for Cardinals, red and gold. There are also saturnos in green for Bishops and red for Cardinals and the Holy Father. In the summer, a version of this hat in straw is sometimes used. The saturno continues the symbolism of the cleric's death to the world and his humble submission to the Church and his clerical duty.
The biretta is used by clerics of at least the order of Sub-Deacons (occasionally by seminarians as well) as both a liturgical and non-liturgical hat. It is worn by clerics during the mass, and even by Bishops and Cardinals while saying a low mass. It is black for clerics through priest, and sometimes has purple or red trimmings for prelates. It is purple for Bishops and red for Cardinals. The Holy Father does not use a biretta, though the Papal hat known as the camauro (red velvet with fur trim) is similar in origin. Also, the academic hats used today share a common origin to the clerical biretta.
The tabarro (Italian for cloak) is a clerical cape primarily for outdoor use in cold weather. For wear over much of clerical attire, a cape is far more practical than a coat. The tabarro in black may be used by all clerics. That of bishops may also be purple, and that of Cardinals and the Pope, red.
The surplice is a white garment, sometime made partly of lace, worn over the cassock while assisting at mass and not in alb (see below), at the Divine Offices, and for other sacraments and rites. It is usually approximately knee-length and is essentially a shortened version of the alb. The Anglican style often has very wide sleeves, while the Roman style tends to have narrower sleeves. Also, the Anglican style sometimes is ankle-length.
The rochet is a garment similar in style to the surplice, but indicative of ecclesiastical jurisdiction. It is proper to Bishops and some prelates. The Roman style is approximately knee-length and has tight sleeves. The Anglican style is ankle-length and has full-sleeves gathered at the wrists with cuffs and ruffles. The Roman style is properly worn underneath the surplice and alb by Bishops and other prelates entitled to the rochet as a symbol of their jurisdiction.
The mozzetta is the element of a Bishop's habit that indicates his jurisdiction and authority as a successor of the Apostles. The mozzetta is purple for Bishops and Archbishops, and red for Cardinals. It is worn over the rochet. It is worn by Bishops only in their own jurisdictions. That is, a Bishop Ordinary wears the mozzetta in his own diocese only. A Metropolitan wears it within his entire Province. A Cardinal wears the mozzetta everywhere. An Auxiliary Bishop, as he is never technically within his own diocese, does not use the mozzetta. Additionally, some Canons use the mozzetta in various colors as part of their Chapter dress. The mozzetta of Bishops in religious orders is usually in a color proper to the habit of that order.
The mantelletta is a symbol of prelatial leadership, but limitation of jurisdiction. The mantelletta is usually purple and worn over the rochet to cover it, thereby symbolizing the limitation of jurisdiction. It is worn by Bishops Ordinary outside their dioceses and by Metropolitans outside their provinces. Bishops Ordinary also wear it underneath the mozzetta when they are in their own diocese, but the Metropolitan or another greater prelate is present. Cardinals wear the mantelletta similarly under the mozzetta when in Rome and the Holy Father is present within the City. As with the mozzetta, some Canons use it as their Chapter dress, and it is in the color of the religious habit for religious bishops. Also, some prelates who are below the rank of Bishop have the mantelletta as their habit, which is why they are sometimes known as "prelates of the mantelletta."
The cappa magna is the most formal element of a Bishop's personal habit. It is a full cape that covers the entire body, the front being usually held up by the wearer's arms, and with a long train. The train is a symbol of jurisdiction, and thus is worn down only in one's own jurisdiction (following the same guidelines as where the mozzetta would be worn.) This garment also has a hood, but the hood is only worn over the head on penitential occasions. The cappa magna is purple for Bishops and red for Cardinals. The Pope makes use of a red cappa magna on a few specific occasions only. The cappa magna is what a Bishop wears for a solemn arrival to a church, for example, to celebrate a solemn liturgy. It furthers the representation of the regal nature of Christ's royal priesthood and the Bishop's role in governing the Church as a successor to the Apostles.
The amice is a rectangular linen cloth worn around the collar and shoulders and over the cassock by the Celebrant, Deacon, and Sub-Deacon (and sometimes some others) before putting on the alb at mass or certain other liturgy. The amice represents the burial cloth placed over the head of our Lord after He was crucified.
The alb is a full-length garment worn over the cassock and amice by the Celebrant, Deacon, and Sub-Deacon (and sometimes some others) at the mass and certain other liturgy. It represents the baptismal garment.
The alb is secured by a rope cincture which, like the fascia (see above), represents chastity.
The maniple is worn over the left sleeve of the alb by the Sacred Ministers at the mass. It is in the liturgical color of the mass and represents service at the altar. Thus it is only worn by Bishops, Priests, Deacons, and Sub-Deacons. The imposition of the maniple is part of the ordination rites for Sub-Deacons.
The stole is worn only by Bishops, priests, and deacons. It represents the yoke of Christ. Deacons wear the stole over the left shoulder as a symbol of their partial ministry. Priests and Bishops wear the stole over both shoulders, as they stand fully with the yoke of Christ as an alter Christus. When worn over an alb, priests cross the ends of the stole in front of them, while Bishops leave the ends to hang straight down. The stole is worn over the alb at mass by the Celebrant and sometimes certain others. It is also worn when prescribed over the surplice, such as when administering a sacrament. For a priest to take the stole symbolizes taking up the yoke of Christ to perform a special sacramental duty.
The dalmatic is the ancient garment of a Deacon. The tunicle is a similar garment used by a Sub-Deacon. The dalmatic is worn over the alb by the Deacon of the Mass, and it is worn over the surplice by Deacons of Honor at a pontifical mass (a mass celebrated by a Bishop). Also, at solemn masses, Bishops wear the dalmatic underneath their chasuble (see below) at mass as a symbol of the fullness of their Holy Orders.
The chasuble is the seamless garment; the garment of Christ's royal priesthood. It represents the garment placed on our Lord during His Passion to mock him. Thus the Celebrant takes the chasuble over the alb to celebrate mass as he, the Celebrant, stands at the altar as an alter Christus. The priest takes the garment used to mock our Lord and turns it into a garment of honor, love, and devotion.
The mitre is the pointed hat worn at solemn mass and certain other occasions by Bishops, Cardinals, the Pope, and certain other prelates who are so entitled. While it likely originated from headwear of Roman officials, its spiritual symbolism is of the descent of the Holy Spirit upon the Apostles at the first Pentecost. Bishops today wear this as a reminder that they continue the burdens of the Apostles.
The pallium is a band of white cloth with crosses embroidered on it worn by Metropolitan Archbishops within their provinces and by the Pope. It symbolizes the image of our Lord carrying the lamb over His shoulder. Often the pallium is secured by or decorated with three pins. These pins represent the three nails used to crucify our Lord. The pallium is only worn over the chasuble.
The crosier is the pastoral staff of a Bishop. Usually in some way shaped like a shepherd's crook, it represents the Bishop's role as shepherd of his flock.