Incest refers to sexual relations between close family members. It is a criminal offence and an impediment to marriage in (almost?) all countries, as well as being against most religions. But the exact definition of what is a "close family member" varies widely: some jurisdictions consider only those related by birth, others also those related by adoption or marriage; some prohibit relations only with immediate family members and ancestors or descendants, while others prohibit relations with aunts and uncles, nephews and nieces, and cousins as well.
All societies place restrictions on who one may marry. Sociology generalizes these restrictions with the terms endogamy -- the group within which one must marry -- and exogamy -- the group one must not marry.
All societies have rules of exogamy, such as incest taboos, that specify ranges and categories of relatives who are forbidden as marriage (and sexual) partners. The most closely related biological kin -- parents, children, brothers and sisters -- are universally included. Most societies restrict other close relatives, but these extensions vary.
Most societies also on specify rules that encourage and sometimes force marriage within groups, frequently ethnic and religious ones. Even in modern Western societies, individuals consistently express preferences for mates from similar class and educational backgrounds, and attempts to violate of this endogamic principle can cause dramatic resistance from the associates of the violators, despite the society's pervasive emphasis on love and individual choice.
Incest is most frequently a form of child sexual abuse, most commonly committed by a father against his daughter. It is universally agreed that this should be illegal.
However, there is also the much rarer phenomena of consensual incestuous relations between adults, such as between an adult brother and sister. This is illegal in most places, but these laws are sometimes questioned on the grounds that such relations do not harm other people and so should not be criminalised. There have been some proposals to repeal these laws, for example, the proposal by the Australian Model Criminal Code Officer's Committee discussion paper "Sexual Offences against the Person" released in November 1996. (This proposal was later withdrawn by the committee, in spite of their own feelings on the issue, due to a large public outcry, which was mostly based on the misunderstanding that the committee was intending to legalize sexual relations between parents and their minor children, which it did not.)