The word literally means "out of communion"; the outward sign of this loss of community involves barring the person from participating in Communion, i.e., receiving the Eucharist, and as a consequence losing their status as members of the church. Certain other rights and privileges normally resulting from membership of the church are revoked. Excommunication is often intended to be only temporary, a "medicinal" procedure intended to provoke repentance. In the Roman Catholic church excommunication is usually terminated by repentance and absolution. For minor excommunications the absolution may be pronounced by a confessor (a Priest). More serious offences must be absolved by a more senior official. - Apostasy, Heresy, or Schism - Desecration of the Eucharist - Physical force against the Pontiff - One who actually procures an abortion and all accomplices - Priest who absolves a partner in adultery - Priest who directly violates the sacramental seal of confession - Both parties to the consecration of a bishop without a pontifical mandate (http://en.wikipedia.org/wiki/Excommunication) Further information can be found in the Catholic Encyclopedia's section on excommunication: (http://www.newadvent.org/cathen/05678a.htm)

I think Duane Roelands gave a decent answer to the first question, but in regards to the last question I can tell you this. A few years ago a Latin American country's (I think it was either Bolivia or Columbia) government legalized abortion. Within a few weeks, the cardinal of that country threatened the entire government with excommunication. Within another few weeks abortion was once again illegalized. Although it wasn't a full excommunication, but it goes to show you that the Catholic Church is still willing to use it when it needs to. There were also rumors that John Kerry was going to be excommunicated during the presidential election of 2004 for being Pro-Choice, but we all know that the Kennedys would never allow that to have happened.

The most common cases are requested by the individuals themselves. See: http://patrickattard.blogspot.com/2009/01/dr-ing-patrick-attard-excommunicates.html