A power reserved to the Pope
The Pope has the power to excommunicate any member of the church, which is to pretty much ban them from the church. If you're banned, then you'll probably go to hell, as the church stops praying for your salvation. Few people were excommunicated, some heretics and Martin Luther, not Madonna.
The power of Interdiction is a special one reserved for the Pope to deal out. The term means suspension of church activities, meaning a priest or church can be prohibited from carrying out mass and ceremonies. A traditional manifestation is the padlocking of a church (thanx Gorgonzola. It is also a special form of excommunication put on entire countries. All church services in the said country are banned, and all the citizens are excommunicated.
It's a powerful political tool that the church used to keep kings in line, and the church strong. AFAIK There's only been 1 interdict placed, and that was on England, during King Henry VIII.
King Henry wanted a divorce, because his wife just wasn't bearing sons. Without an heir to the throne, the horrendous war of the roses between the Tudors and Plantagenets over the throne would begin again. So he asked the Pope for an annullment, a sanctioned divorce. The Pope couldn't because Anne Boulin's uncle placed troops in the Vatican to threaten the Pope. So the Pope, caught between a rock and a hard place, said "I'll think about it and get back to you."
Henry, tired of waiting for years, threw up his hands and claimed that since he was ruler of England he should be the person controlling England's churches. This was the start of the Anglican Church. The Pope, furious at Henry's impertinence, issued the order of the Interdict, saying all the British will go to hell unless Henry stopped immediately. I imagine the citizens were all angry over this, probably more at the king than the Pope.
Gorgonzola says http://www.newadvent.org/cathen/08073a.htm goes on at quite some length about it.