In general it means the united body of faithful Christians–the mystical Body of Christ. The Catholic Church is that body and it includes the Church millitant on earth, the Church suffering in Purgatory and the Church triumphant in Heaven. It also includes those non-Catholics (who don’t profess Catholic Christianity through no fault of their own) who are united in a mystical way to the Body.
It can also refer to the hierarchy or a local community (ie the church at Antioch).
Of course, it can also mean the building where Mass is offered.
