Put simply, the Nicene Creed states a belief in "one holy, catholic, and apostolic church," and the concept of an apostolic church (i.e., one in the apostolic succession as the Roman Catholic Church defines it, in which each bishop is ordained in an unbroken line reaching back to the Apostles) is key to which churches are capital-C Churches according to the Roman Catholic Church and which are ecclesial communities.
Cf. Responses to Some Questions Regarding Certain Aspects of the Doctrine on the Church, June 29, 2007 | CDF:
Why does the Second Vatican Council use the term
“Church” in reference to the oriental Churches separated from full
communion with the Catholic Church?
The Council wanted to adopt the traditional use of the term.
“Because these Churches, although separated, have true sacraments and
above all – because of the apostolic succession – the priesthood and
the Eucharist, by means of which they remain linked to us by very
close bonds”, they merit the title of “particular or local
Churches”, and are called sister Churches of the particular
This category covers the Eastern Orthodox Churches, the Oriental Orthodox Churches, and a handful of smaller groups such as the Church of the East in which the RCC recognizes a valid line of Apostolic succession and more-or-less "correct" sacraments, but nevertheless are not in communion with Rome.
Why do the texts of the Council and those of the Magisterium since the
Council not use the title of “Church” with regard to those Christian
Communities born out of the Reformation of the sixteenth century?
According to Catholic doctrine, these Communities do not enjoy
apostolic succession in the sacrament of Orders, and are, therefore,
deprived of a constitutive element of the Church. These ecclesial
Communities which, specifically because of the absence of the
sacramental priesthood, have not preserved the genuine and integral
substance of the Eucharistic Mystery cannot, according to Catholic
doctrine, be called “Churches” in the proper sense.
The classic example of one of these bodies would be the Anglican Communion, which considers itself a church in the Apostolic Succession but for a variety of reasons, including the sacramental form/intent of ordination, is considered an ecclesial community by the Vatican.
However, "ecclesial" means "pertaining to church," and it seems strange that "ecclesial communities" translates to "church communities" but ecclesial communities are not "Churches" under the Roman understanding. It would appear the difference is one of degree; as the response to the Fourth Question above notes, the capital-C Churches have a similar understanding of the sacraments, the apostolic succession, the priesthood, and the Eucharist as the Body and Blood of Christ despite not being in full communion with Rome, while the answer to the Fifth Question points out that the Reformation churches specifically have a much different understanding of the sacraments, the apostolic succession/priesthood, and the Eucharist in addition to lacking communion with Rome. In other words, the RCC recognizes any Trinitarian baptism performed in the name of the Father, the Son, and the Holy Spirit, and it is clear groups of people baptized under a Trinitarian formula deserve to be recognized as something, but not as a traditional capital-C Church in the same sense as it recognizes the Eastern Orthodox, et al.
The term "ecclesial community" can also refer to "ecclesial movements" (lay associations of the faithful
) like Communion & Liberation, Opus Dei, Neocatechemenal Way, etc.