From YOUCAT, fact 255:
255: What happens in diaconal ordination?
In diaconal ordination the candidate is appointed to a special service within the sacrament of Holy Orders. For he represents Christ as the one who came, “not to be served but to serve, and to give his life as a ransom for many” (Mt. 20:28). In the liturgy of ordination we read: “As a minister of the Word, of the altar, and of charity, the DEACON will make himself a servant to all.”
The original model of the deacon is the martyr St. Stephen. When the APOSTLES in the original Church of Jerusalem saw that they were overwhelmed by their many charitable duties, they appointed seven men “to serve tables,” whom they then ordained. The first mentioned is Stephen: “full of grace and power,” he accomplished much for the new faith and for the poor in the Christian community. Over the centuries the diaconate became merely a degree of Holy Orders on the way to the presbyterate, but today it is once again an independent vocation for both celibates and married men. On the one hand, this is supposed to reemphasize service as a characteristic of the Church; on the other hand, it helps the priests, as in the early Church, by establishing an order of ministers who take on particular pastoral and social duties of the Church. Diaconal ordination, too, makes a lifelong, irrevocable mark on the ordained man.