A Cathederal is a church in which a bishop or higher ordained clergy preaches.
In Anglicanism, this is the church which houses the cathedra or ceremonial throne of the bishop of the diocese. The cathedral congregation is under the spiritual care and leadership of a priest called the Dean, and under the governance of a chapter which is roughly parallel to the vestry of a parish church.
Although cathedrals are often great in size and architecture, neither of these things makes a church a cathedral—many are quite small.