What sins can be absolved only by a bishop in the Roman Catholic Church? My understanding is that only the Pope can give absolution to a priest who reveals the content of someone's confession.

What sins can be absolved only by a bishop or by a priest the bishop specifically appoints?
A priest can absolve any sin; however, a priest cannot absolve from reserved excommunications unless he is given permission to do so by his bishop (cf. the rite for absolving from excommunication).

Some sins (e.g., abortion; cf. Ken Graham's answer above) also incur an excommunication that is reserved to the local ordinary (bishop), and other excommunications (e.g., for illicitly consecrating bishops, as happened in China) are even reserved to the Apostolic See (pope). Thus, while a priest could absolve the sin, a bishop or pope might be necessary to absolve the excommunication (i.e., reincorporate the penitent back as a member of the Church).