Who is a deacon
The life of the first Christian community is narrated in the book of the Acts of the Apostles. In Jerusalem this scene, considered as a precedent for the current figure of the deacons in the Church takes place:
At that time, as the number of disciples continued to grow, the Hellenists (Christians) complained against the Hebrews (Christians) because their widows were being neglected in the daily distribution. So the Twelve called together the community of the disciples and said, "It is not right for us to neglect the word of God to serve at table. Brothers, select from among you seven reputable men, filled with the Spirit and wisdom, whom we shall appoint to this task, whereas we shall devote ourselves to prayer and to the ministry of the word." The proposal was acceptable to the whole community, so they chose Stephen, a man filled with faith and the holy Spirit, also Philip, Prochorus, Nicanor, Timon, Parmenas, and Nicholas of Antioch, a convert to Judaism. They presented these men to the apostles who prayed and laid hands on them. (Acts 6: 1-6)
These are the first seven deacons (which comes from the Greek "διάκονος", "diakonos", meaning "one who serves")
Here are also some quotes from the Catechism of the Catholic Church (nn. 1569-1570)
1569 "At a lower level of the hierarchy are to be found deacons, who receive the imposition of hands 'not unto the priesthood, but unto the ministry."'At an ordination to the diaconate only the bishop lays hands on the candidate, thus signifying the deacon's special attachment to the bishop in the tasks of his "diakonia."
1570 Deacons share in Christ's mission and grace in a special way. The sacrament of Holy Orders marks them with an imprint (“character") which cannot be removed and which configures them to Christ, who made himself the "deacon" or servant of all. Among other tasks, it is the task of deacons to assist the bishop and priests in the celebration of the divine mysteries, above all the Eucharist, in the distribution of Holy Communion, in assisting at and blessing marriages, in the proclamation of the Gospel and preaching, in presiding over funerals, and in dedicating themselves to the various ministries of charity.