Since there are no deacons (or priests) who are answering this question, I, a lay man, will hazard an answer.
A deacon is one of the three orders of the Sacrament of Holy Orders (diaconate, presbyterate, episcopate—these latter two are the two ranks of priesthood) which we Catholics consider to be of either divine or apostolic origins. According to the Book of Acts (?), the two groups of Jews, the Greeks and the…(?)…were quarreling among themselves about the aid given to each. It was decided that seven men should be chosen among themselves, whom the apostles will appoint (and pray over) to supervise the table service and the charitable aid, so that the apostles will devote themselves to ministry. Anyway, to make a long story short, these are considered to be the first deacons.
Now, for the specifics of diaconia in today’s world:
A deacon (in the Latin Church) is authorized to baptize, to witness marriages on behalf of the Church (when there is no Mass), to officiate over burials, to expose the Blessed Sacrament as well as give Benediction, to bring Holy Communion to the infirm, to proclaim the Holy Gospel, and to preach. He is ordained to the ministry, but not the ministerial priesthood (all the baptized are called to the priesthood of the faithful, who exercise their legitimate roles in the celebration of the Mass) which is necessary to celebrate the Eucharist (in other words, no ordained priest, no Mass). As such, he is a cleric due to sacred ordination and no longer a layperson. In all cases, he is to be a man with deep reverence for the sacred and to be a man of prayer, always living in a way that is not unbecoming of the calling which he has received (I should also add that we lay people should also live in a fashion which brings praise to God, keeping our baptismal dignity unstained).
In the eastern Catholic Churches, I believe that the last two are the only things authorized out of the above list (with the exception of emergency baptisms), but they also have an extensive role in the Divine Liturgy (or the Mass in the Latin Church). For example, I think it is the deacon who calls the worshippers attention when necessary or as part of the liturgy (“Wisdom, be attentive!”). But the part of the eastern Churches I know only from second hand knowledge, so I yield to those who have better knowledge than me.
That being said, being a diocesan (or eparchial) deacon or priest is a separate call from the one to the religious or consecrated state, though in a few individuals they may coexist. I hope this helps.
With blessings,
Paul, a fellow servant of His Majesty our God and King (and a grievous sinner)