The typical Protestant conception of the Church is that it is invisible. Though individuals may group together for fellowship and Bible study, their churches are really like clubs in a city. The real church, say Protestants, is the broad and unseen group of the saved.
The Catholic Church, in contrast, teaches that the Church is a visible organization. Being a visible organization, it can be identified--it has marks. The marks are that it is one, holy, catholic, and apostolic--"one" in that it is a unified organization, "holy" in that it is an organization divinely established, "catholic" in that it is to embrace all of mankind, and "apostolic" in that a line of succession has been kept with the authority Christ passed to Peter and the apostles.