Ecclesiastical Latin, sometimes called "Church Latin", is the Latin language as used in documents of the Roman Catholic Church.
The dogmatic definitions of the first seven General Councils were given in Greek, and even in Rome Greek was at first the language of the liturgy and the language in which the first Popes wrote. Clearly, the Holy See is not obliged to have Latin as its official language and, in theory, could change its practice.
However, this is unlikely to happen in the foreseeable future. As a language no longer in common use (a "dead" language, though some would dispute the exactness of this description), Latin has the advantage that the meaning of its words is no longer subject to change from century to century. This helps to ensure theological precision and to safeguard orthodoxy. Accordingly, Pope John Paul II has reaffirmed the importance of Latin for the Church and in particular for those doing ecclesiastical studies.
Especially since the Second Vatican Council, Latin is no longer the exclusive language of the liturgy of the Roman and Ambrosian rites of the Catholic Church – by 1913 the Catholic Encyclopedia was already commenting on beginnings of the replacement of Latin by vernacular languages – but official liturgical texts are still produced in Latin, thus providing a clear single point of reference for translations into all languages.
The same holds for the official texts of canon law.
Since Latin ceased to be an everyday language even among scholars, papal documents and the like have for some centuries usually been drafted in a modern language, but the authoritative text, the one published in the Acta Apostolicae Sedis, is generally in Latin, even if this text becomes available only later.
The Catechism of the Catholic Church was drafted in French and appeared first in that language. But five years later, when the Latin text appeared, the French text had to be corrected in line with the Latin version.
Occasionally, the official texts are in a modern language. Two of the best known are Tra le sollecitudini  (http:/www.adoremus.org/MotuProprio.html) (1903) by Pope Pius X, in Italian, and Mit brennender Sorge (1937) by Pope Pius XI, in German.
The rule now in force on the use of Latin in the eucharistic liturgy of the Roman rite is: "Mass is celebrated either in Latin or in another language, provided that liturgical texts are used which have been approved according to the norm of law. Except in the case of celebrations of the Mass that are scheduled by the ecclesiastical authorities to take place in the language of the people, Priests are always and everywhere permitted to celebrate Mass in Latin" (Redemptionis Sacramentum, 112). (http://www.vatican.va/roman_curia/congregations/ccdds/documents/rc_con_ccdds_doc_20040423_redemptionis-sacramentum_en.html)
Ecclesiastical Latin is not a different language from classical Latin. Study of the language of Cicero and Virgil is quite sufficient for understanding Church Latin. However, those interested only in ecclesiastical texts may prefer to limit the time they devote to ancient authors, whose vocabulary covers matters that, though of importance in that period, are unlikely to be dealt with in Church documents.
Ecclesiastical Latin is in most countries pronounced (http://community.middlebury.edu/~harris/LatinBackground/Pronunciation.html) as is traditional in Rome, giving the letters the value they have in modern Italian, but without distinguishing between open and close "e" and "o". However, ecclesiastics in some countries follow slightly different traditions. For instance, in Slavic countries and in German-speaking ones the letter "c" before the front vowels "e" and "i" is commonly given the value represented in English by "ts", and "g" in all positions is pronounced hard, never as English "j". (See also: Latin regional pronunciation)
The complete text of the Bible in Latin can be found at Nova Vulgata - Bibliorum Sacrorum Editio (http://www.vatican.va/archive/bible/nova_vulgata/documents/nova-vulgata_index_lt.html). A site (http://faculty.acu.edu/~goebeld/vulgata/newtest/vnt.htm) of more uncertain permanence gives, side by side, the Vulgate Latin text of the New Testament and the Rheims English translation.
A Vatican institution, the Latinitas Foundation (http://www.vatican.va/roman_curia/institutions_connected/latinitas/documents/index_lt.htm), exists to promote the use of Latin not only in Church documents but in all facets of modern life.
Among its initiatives has been the publication of the 15,000-word Lexicon Recentis Latinitatis, which indicates Latin terms to use in referring to a bicycle (birota), a cigarette (fistula nicotiana), a computer (instrumentum computatorium), a cowboy (armentarius), a motel (deversorium autocineticum), shampoo (capitilavium), a strike (operistitium), a terrorist (tromocrates), a trademark (ergasterii nota), an unemployed person (invite otiosus), a waltz (chorea Vindobonensis), and even a miniskirt (tunicula minima) and hot pants (brevissimae bracae femineae). Some 600 such terms can be seen on a page (http://www.vatican.va/roman_curia/institutions_connected/latinitas/documents/rc_latinitas_20040601_lexicon_it.html) of the Vatican website. (http:/www.vatican.va)
- Ecclesiastical Latin (http://www.newadvent.org/cathen/09019a.htm) (article in Catholic Encyclopedia (1913) (http://www.newadvent.org/cathen/))
- Veterum Sapientia (http://www.adoremus.org/VeterumSapientia.html) by Pope John XXIII, 1962
- What the Church says on the Latin Language (http://home.earthlink.net/~thesaurus/thesaurus/Popes.html)
- The Necessity of Latin (http://www.traditio.com/tradlib/latneces.txt) (a collection of quotes from Popes, councils, and saints)
- Latin-English pre-Vatican-II Breviary (http://www.breviary.net/)
- Ordo Missae of the 1970 Roman Missal (http://www.catholicliturgy.com/index.cfm/FuseAction/TextContents/Index/4/SubIndex/67/TextIndex/9), Latin and English texts, rubrics in English only
- The New Missal Latin by Edmund J. Baumeister, S.M., Ph.D. Published by St. Mary's Publishing Company, P.O. Box 134, St. Mary's, KS 66536-0134, USA