The Book of Leviticus is the third book in the Old Testament of the Bible. It is, essentially, the Mosaic Law, a series of laws laid down by Yahweh through Moses. These include what not to eat, how and when to perform animal sacrifices to the Lord, what is clean and unclean, and what kinds of sexual activities are worthy of the death penalty.
The book of Leviticus is largely ignored by modern Christians, although it has greater significance to Jews as the source of many rules still observed today. The one context in which Leviticus is commonly quoted by Christians is when fundamentalists are espousing their views on sexual purity. In such cases the rules about menstruation and nocturnal emissions are always ignored.
There are a few "good" rules in Leviticus, such as Leviticus 19:18 which says to love one's neighbor as oneself. However, these sorts of general philosophical rules are rare, and usually applicable only to behavior towards the Israelites and (sometimes) foreigners who live with the Israelites and observe similar customs (hence "love your neighbor" and not "love your fellow man").
Furthermore, these rules are interspersed with laws about ritual or sexual purity that most would now find absurd. Immediately after the "love thy neighbor" bit, Leviticus 19:19 insists that one may not mix different types of animals, seeds, or clothing. Leviticus 20 contains rules about how closely a man's wives can be related to each other, about killing animals that have participated in bestiality, about exiling any couple that has sex while the woman is menstruating, and about killing any man who has gay sex (but only men).