Avodah Zarah (Hebrew: "foreign worship," meaning "idolatry" or "strange worship") is the name of a tractate in the Talmud, located in Nezikin, the fourth Order of the Talmud dealing with damages. The main topic of the tractate is laws pertaining to Jews living amongst Gentiles, including regulations about the interaction between Jews and "idolaters" (who represented most of the Gentile population during the time of the Talmud), both for distancing Jews from the act of idolatry in every shape as well as distancing Jews from the threat of cultural assimilation.
Because of its subject matter, Avodah Zarah has probably been the most controversial tractate in the Talmud, and historically it has been the subject of criticism from the Christian Church. Christians view the polemic of the tractate as being directed at them, and as painting them as idolaters and immoral people. The traditional Orthodox Jewish attitude has been that the tractate was authored in times of the Roman persecution and that this is what is generally meant by "idolatry" in the tractate.
The placing of the tractate in the order of "damages" says something about the attitude of the Sages towards idolatry. By engaging in idolatry, a person is seen as in effect denying God's creation and doing actual damage to the spiritual forces of creation. It also has the effect of removing the action of idolatry from a purely abstract notion into a real transgression with tangible effects and punishments, alongside theft and swearing a false oath.