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For men only -

Only men can become Catholic priests. The fundamental reason for this is historical: the male-only priesthood is grounded in the events of the Last Supper.

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The Last Supper -

Jesus chose only men in selecting the Twelve Apostles, and the Church feels bound by that choice made by the Messiah.

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Vow of celibacy -

A Catholic priest is required to take a vow of celibacy. Celibacy became mandatory for Latin Church priests in the 11th century.

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Renunciation of marriage -

Roman Catholic priests aren't allowed to get married. But this rule only applies throughout the Western Catholic discipline.

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Ordaining of married men -

The Eastern Catholic discipline, on the other hand, allows the ordaining of married men. But once ordained and if widowed, a priest is not allowed to remarry and is required to live celibately thereafter.

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Being defrocked -

A defrocked Catholic priest, one who is laicized or deprived of ecclesiastical status, still must observe celibacy.

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Children of the ordained -

If a priest breaks the rules and fathers a child, Vatican guidelines stipulate that his responsibility as a dad overrides his duties to God. He's therefore expected to request laicization and embrace the role of parent.

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Reading the Liturgy of the Hours -

Roman Catholic priests and monks are early risers. They start the day at 3 am to pray the Liturgy of the Hours, a form of prayer that follows the rhythm of each day through psalms, Scripture, and prayers, and an obligatory way of sanctifying the day.

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Marking the hours of each day -

Morning Prayer is followed by Daytime Prayers, said mid-morning, noon, and in the afternoon. Evening Prayer is said at the end of the day before bedtime.

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The Seal of Confession -

The Seal of Confession is a priest's silence after hearing the sins of a penitent. The Seal of Confession is considered absolutely inviolate.

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A priest can never break the Seal of Confession -

A priest who hears the Confession must not, under any circumstances, reveal the confessed sins of the particular penitent ever, even to law enforcement.

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A priest cannot absolve his own sins -

Priests are human. They can make mistakes. But they cannot absolve their own sins. Instead, they must go to another priest to receive the sacrament of Reconciliation.

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Cardinals over 80 cannot vote for a pope -

Cardinals are the highest-ranking ecclesiastical officials after the pope. They form the Sacred College, which elects succeeding pontiffs.

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A state of mind -

Cardinals are responsible for secretly electing the next pope when one dies, but only if they are under 80 years old. Octogenarians are not allowed to cast a vote, apparently because older cardinals are not in the right state of mind to take part in the election process, according to the papal document Ingravescentem aetatem.

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Some priests have to shave their heads -

In some quarters of the Catholic Church, tonsure, the practice of cutting or shaving some or all of the hair on the scalp, is mandatory for new priests and monks.

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The practice of tonsure -

Despite the practice being largely abandoned by papal order in 1972, tonsure is still undertaken by Catholic Traditionalists—groups of priests who still prefer to observe the old Latin Mass instead of the current Novus Ordo (new order of the Mass).

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Canon Law governing the Eucharist -

According to canon 905 of the Code of Canon Law, a priest is not permitted to celebrate the Eucharist more than once a day.

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A singular event -

The reason for this is twofold. Firstly, by celebrating Mass just once, the activity maintains its integrity as a special occasion.

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Open to financial abuse -

The second reason is to deter unscrupulous priests from helping themselves to multiple parish donations were the Eucharist held several times a day. During Mass it's commonplace for the congregation to give donations, some of which is spent by the priest for good causes. Multiplying this generosity could tempt financial abuse, so the Vatican closed the loophole by restricting celebration of the Eucharist to no more than once a day.

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Revering Jesus' body and blood -

The Eucharist—the consecration and consumption of bread and wine—is at the heart of Christian worship. Once consecrated, those mini communion wafers known as hosts are the literal body and blood of Jesus Christ.

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The transubstantiation -

As the literal body and blood of Jesus Christ (a doctrine known as transubstantiation), the wafers must be treated with great reverence. A priest can never throw them away.

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Priests can imbibe alcohol -

Surprisingly, perhaps, priests are allowed to enjoy alcoholic beverages. They don't profess any vows that prohibit them from drinking.

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Priests can show sinners the door -

A priest will refuse to conduct communion with a Catholic if they've committed any serious sins since their last confession.

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Rules for becoming a priest -

Besides being an unmarried male, there are a number of additional criteria to meet if you're considering joining the Roman Catholic priesthood. 

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Minimum requirements -

You must be at least 25 years old and have been baptized.

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Confirmation of intent -

Meeting the basic requirements to become a priest also includes having already undertaken confirmation. Confirmation is one of the seven sacraments of the Roman Catholic, Old Catholic, and Orthodox churches, and a sacramental act in the Anglican church.

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Separation issues -

You must not have gone through a divorce or must annul your marriage if you are separated.

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Time to gather your thoughts -

If contemplating the priesthood as a widower, two years must pass since your loss to allow time for mourning before making priestly promises.

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Role of the diocesan priest -

A diocesan priest, one who normally serves within the boundaries of his diocese under the authority of his bishop, does not make the solemn vows that religious priests make, though he does make promises of obedience and celibacy.

Sources: (The Holy See) (Catholic World Mission) (EWTN) (Tekton Ministries) (Vault) 

See also: Secrets the Catholic Church tried to hide

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