Cardinals are appointed by the Pope to form a special 'College of Cardinals' whose main task is to elect a new Pope, when that office becomes vacant, in a meeting called a 'conclave'. They are also sometimes asked to represent the Pope at special events.
The Pope periodically calls a meeting of the College of Cardinals, called a Consistory'. Consistories are normally called when new cardinals are created.
Church law (canon 351) says that Cardinals are bishops, "outstanding in doctrine, virtue, piety and prudence in practical matters". Cardinals of 80 years of age or older cannot vote in a conclave, and the maximum number of electors at any time is 120 (John Paul II, Universi Dominici Gregis, 1996).