A transitional deacon will typically become a priest, while permanent deacons usually do not.
There are two ways of fulfilling the order of deacons in the Catholic Church. One way is as a “transitional” deacon and the other is as a “permanent” deacon.
Sometimes the bishop will ordain both transitional and permanent deacons at the same ceremony, which can make it difficult to tell the difference.
First of all, the USCCB provides a succinct definition of a deacon and his particular role in the Church.
“A deacon is an ordained minister of the Catholic Church. There are three groups, or ‘orders,’ of ordained ministers in the Church: bishops, presbyters and deacons. Deacons are ordained as a sacramental sign to the Church and to the world of Christ, who came ‘to serve and not to be served.’ The entire Church is called by Christ to serve, and the deacon, in virtue of his sacramental ordination and through his various ministries, is to be a servant in a servant-Church.”
The diaconate is the first “order” in the sacrament of holy orders, and for those called to the priesthood it is the first order they receive.
All men on their way to the priesthood must first be ordained a deacon. This is called the “transitional” diaconate, because it marks a transition to the presbyterate.
However, not all deacons become priests.
Some men are called by God to serve the Church in a way that does not involve the priesthood. This permanent diaconate is a ministry of service, both liturgically (primarily through preaching, acting as an ordinary minister of holy communion, and celebrating baptisms and officiating at weddings and funerals held outside a Mass), and in the many service ministries of parish and diocesan life. Married men may be ordained to the permanent diaconate.
In this case, permanent deacons will always be deacons, though in rare circumstances (often after the death of a spouse) they may be ordained to the priesthood if the bishop gives special permission.
The vocation to the permanent diaconate is a special call in the Church and remains a powerful channel of grace for those men who desire to serve the Church without becoming a priest.