What is a cloisters in a church?

It is a covered walk in a religious institution having an open arcade opening onto a courtyard. There will be a wall around the outside of the cloister

In the christian belief, it isn't the building that is the church; A church is the body of believers, who meet within the building.

Cloisters were for monks to walk during bad weather. If a church or cathedral doesn't have cloisters it means that it never had monks attached to it - or perhaps the cloisters were destroyed during...

The plural noun for church is churches.

No, Orthodox churches are where persons of Greek or Russian Orthodox faith worship. Like the Church of England, the Orthodox also separated from the Roman Catholic Church on matters of doctrine.