Sacrilege (from the Latin sacrilegium, meaning “temple robber”) is the sin of profaning any religious item or any person, place, or thing consecrated to God or to His Church (Catechism #2120). Desecrating a church or house of worship is one of the worst kinds of sacrilege, but the most grievous and heinous act of sacrilege is the desecration of the Blessed Sacrament (also called the Holy Eucharist or Holy Communion, the consecrated wafers of bread kept in the tabernacle in church) since Catholics firmly believe it is the real Body and Blood, Soul and Divinity of Christ.
Canon #1367 of the 1983 Code of Canon Law imposes an automatic excommunication on anyone who desecrates the Blessed Sacrament, and only the Pope can remove this penalty after one repents. Often, political activists will commit sacrilege by desecrating the Host (consecrated wafer) to make a political statement against the Catholic Church. That action is as obscene to Catholics as would be the spray painting of swastikas inside a Jewish synagogue or the burning of a Bible to a Fundamentalist Protestant.
Sacrilege can be personal, local, or real. Personal sacrilege is irreverence shown to a person consecrated by religious vows (monks, nuns, etc.) or by holy orders (deacons, priests, bishops). Ridiculing, mocking, or abusing members of the clergy is considered personal sacrilege, as often the animosity is directed not at the person themselves but at the Church or at God whom they represent. Whenever those in religious or clerical life violate the sixth Commandment and break their vow of chastity, it is considered a personal sacrilege on their part. Laying violent hands on a cleric used to incur an automatic excommunication from the 1917 Code of Canon Law. Since 1983, only someone who physically attacks the pope is excommunicated.
Local sacrilege is the violation and desecration of sacred places and space. Robbing or vandalizing a church, chapel, oratory, convent, or monastery would be of this category. It could also be committing immoral and sinful acts inside a sacred building, such as committing murder or engaging in sexual acts. The previous law (1917 Code) considered the burial of a publicly excommunicated person in a Catholic cemetery or blessed grave to be sacrilege. The current law (1983 Code) makes no mention of it.
Real sacrilege is the contemptuous irreverence shown for sacred things, especially the seven sacraments or anything used for divine worship (altars, vestments, chalices, tabernacles, et al). Using sacred vessels for secular use, such as a chalice to drink cocktails, or using common items like paper plates and Styrofoam cups for liturgical worship, are also examples of real sacrilege. The worst kind, again, is the desecration of the Blessed Sacrament, as it is the most important and most sacred item in Catholicism (far more than any relic or historical artifact whatsoever).