Excommunication is normally associated with the Roman Catholic Church but other Sects and denominations do it as well.
Roman Catholics[]
Being excommunicated among Roman Catholics means that a person can't officially take part in the sacraments of the church. Penitent people who have been excommunicated can still receive the sacrament of confession and absolution sometimes with restrictions. [1] Furthermore according to Roman Catholics the excommunicated person must repent of whatever caused the excommunication or face Terrifying divine Punishment as taught by Roman Catholics after death.
Others[]
In some other Religious groups excommunication leads to the excommunicated person being denied social contact with the group. This can be so extreme that even close family members are denied contact with anyone who has been excommunicated. Jehovah's Witnesses sometimes force family members to stop contact with someone who leaves, though practise varies. This can be called disfellowshipping or shunning, different groups have different names. In Scientology the term is disconnection. When a person's social network centres round being forced to break contact with former friends is seriously troubling.
There is a good and a a bad side to Christianity, see the category page