A deconsecrated church in general is a surviving church building which is no longer usable for worship.
In the Roman Catholic Church, however, strictly speaking the term is reserved for a former church which has ben formally abandoned. This involves a process in Canon Law involving the Superior in charge (almost always a bishop or the head of a religious congregation), and requires the removal of sacred items such as altar relics (often the altar mensa is removed or broken).
Hence, in the Catholic Church there is a distinction between a "deconsecrated church" and a "desecrated church". The latter is a building that has been rendered unfit for worship in some way, without formal deconsecration. The end result is the same, however, and so no attempt is made to distinguish the two terms in this Wiki.
The old rule used to be that a Catholic church had to have Mass celebrated on its main altar at least once a year. There are, however, "dark" churches in Rome which are closed down without having been deconsecrated (the most obvious example in the Centro Storico is San Francesco di Paola ai Monti).
There are several reasons why a church may be formally Deconsecrated:
Lack of repair to the building. which renders it dangerous or impossible to use -in this case, the church is usually reconsecrated after rebuilding or repair -e.g. San Pier Damiani.
The building being too small for its congregation, leading to a new church being built alongside or in the locality -e.g. Santa Maria del Carmine e San Giuseppe al Casaletto, San Benedetto al Gazometro. Tiny farmstead chapels in developing suburbs are very vulnerable to this -e.g. San Clemente al Castello di Torrenova.
The building being too large for its congregation. In the more distant past, some Roman churches were deconsecrated, reduced in size and reconsecrated -e.g. San Vitale.
The institution attached to the church is shut down. In Rome this was often a convent -e.g. Santa Maria Annunziata delle Turchine, but could be a confraternity (Santa Maria Addolorata dei Sacconi Rossi), guild (Sant'Andrea dei Vascellari) or school (Sacro Cuore di Gesù della Scuola Pontificia Pio IX).
A surprising number of Centro Storico churches were deconsecrated because the institution attached wished to escape to the suburbs. This seems usually to have been a temptation of houses of study or seminaries -e.g. Sant'Andrea degli Scozzesi, San Lorenzo da Brindisi a Ludovisi.
A speciality at Rome is the church which was deconsecrated because "archaeologists" wished to "purify" ancient structures from later fabric -e.g. Sant'Adriano, Santa Maria del Sole, Santa Maria Egiziaca.
A church is Desecrated by sudden serious damage, such as a fire, flood or explosion (San Giorgio in Velabro). Also (a very old rule) any church in which human blood is shed is desecrated, or any into which filth has been brought. The instances of Roman churches used as stables by enemy troops illustrates this last method, as horse shit on the floor would constitute desecration -Santa Maria in Aracoeli. Further, any church in which an anti-Christian liturgical rite is celebrated (a Satanic celebration, for example -see San Ciriaco in the page for Santi Quirico e Giulitta) is considered desecrated.
Or, a church or chapel could simply be abandoned and allowed to go derelict without anybody being bothered to deconsecrate it. There are few examples of this in Rome compared to other cities in Italy such as Palermo or Naples, but there are some -e.g. Santa Chiara a Villa York. Private country estate chapels are especially vulnerable.
For a list, see category page "Deconsecrated Churches" (link below).