The diaconate is one of the three "higher ordinations" of the Church. In the course of the centuries it lost its own sacramental character; the ordination was considered in the long run as a mere preparation to priesthood. Vaticanum II (1962-1965) corrected this, re-establishing it as an independent ministry. This renewed diaconate has been described in the Dogmatic Constitution on the Church: "Lumen gentium". The key text is written in par. 29: http://www.vatican.va/archive/hist_councils/ii_vatican_council/documents/vat-ii_const_19641121_lumen-gentium_en.html

29. At a lower level of the hierarchy are deacons, upon whom hands are imposed "not unto the priesthood, but unto a ministry of service."(74*) For strengthened by sacramental grace, in communion with the bishop and his group of priests they serve in the diaconate of the liturgy, of the word, and of charity to the people of God. It is the duty of the deacon, according as it shall have been assigned to him by competent authority, to administer baptism solemnly, to be custodian and dispenser of the Eucharist, to assist at and bless marriages in the name of the Church, to bring Viaticum to the dying, to read the Sacred Scripture to the faithful, to instruct and exhort the people, to preside over the worship and prayer of the faithful, to administer sacramentals, to officiate at funeral and burial services. Dedicated to duties of charity and of administration, let deacons be mindful of the admonition of Blessed Polycarp: "Be merciful, diligent, walking according to the truth of the Lord, who became the servant of all."(75*)

Since these duties, so very necessary to the life of the Church, can be fulfilled only with difficulty in many regions in accordance with the discipline of the Latin Church as it exists today, the diaconate can in the future be restored as a proper and permanent rank of the hierarchy. It pertains to the competent territorial bodies of bishops, of one kind or another, with the approval of the Supreme Pontiff, to decide whether and where it is opportune for such deacons to be established for the care of souls. With the consent of the Roman Pontiff, this diaconate can, in the future, be conferred upon men of more mature age, even upon those living in the married state. It may also be conferred upon suitable young men, for whom the law of celibacy must remain intact.

Another important and revealing study on the diaconate in the early days of the Church is:

Diakonia: Re-interpreting the Ancient Sources, John N. Collins. Oxford: Oxford University Press, 1990. Pp. xi, 368. ISBN 0-19-506067-9.

The Catholic Encyclopedia also offers information about deacons and diaconate on: http://www.newadvent.org/cathen/04647c.htm