God commanded the implementation of a tithing system for ancient Israel, based on a person's "increase" (Deuteronomy 14:22), in order to provide support for the tribe of Levi (Numbers 18, Deuteronomy 18). This tribe was dedicated by the Eternal (Numbers 8) to serve the religious needs of the entire nation (e.g. serve in and maintain the portable tabernacle, which later became permanent in the form of Jerusalem's temple). This support through tithing, along with offerings, was needed since the Levities did not receive an inheritance of farmable land when Israel entered the Promised Land (Numbers 18:20 - 21, Joshua 18:7, etc.).
Modern tithing is the practice of a person giving a tenth of their "increase" to support a religious purpose such as preaching the gospel, support of paid pastors, church-related expenses, and so on.
Churches and fellowships vary widely on their views and practices regarding this Biblical teaching. Some groups teach that although the practice was commanded in the Old Testament (Leviticus 27:30 - 33, Numbers 18:20 - 32, Deuteronomy 12:5 - 6, 14:22 - 23, etc.) it was considered no longer valid in the New Testament. Others argue that the practice was not abrogated in the New Testament church and that it is in force today for Christians.
Some that believe in tithing teach that it should be based on a person's gross income (before taxes and other deductions) while others state that a tenth should be given based on one's net income (income after taxes, expenses related to a person's employment or business, etc.).
Concerning the implementation of this doctrine, some churches leave it up to the conscience of individual members to determine what is their "increase" on which tithing is based. Other organizations, however, dictate to their membership when and on what they should give and go so far as strictly enforcing their rules through their ordained ministry.
This practice is considered, by some religious groups, as such a critical gauge of a person's dedication and loyalty that contribution amounts are often checked to determine whether a particular person is offered a leadership position within the church (e.g. deacons, elders, pastors, etc.). The absence of what is considered an expected level of support may also be used as grounds for excommunicating (disfellowshipping) a person from a fellowship.
The Old Testament also speaks of a second and likely third tithe that was commanded. In addition to giving 10% each year for the support of the Levitical priesthood, a second 10% was to be saved and personally kept to pay expenses (travel, food, lodging, etc.) for observing God's annual Feast Days (especially the Feast of Tabernacles - see Deuteronomy 14:22 - 27). Modern churches and fellowships who endorse the keeping of God's Holy Days usually teach the setting aside of this tithe.
Tithing an additional ten percent beyond the total twenty percent set aside annually was also commanded (also known as the "poor tithe") to be done every third and sixth year in a seven year cycle (Deuteronomy 14:28 - 29), a fact confirmed by the Jewish historian Josephus (Antiquities of the Jews, Book 4, Chapter 8, Section 22). This contribution was given to "the Levite, the stranger, the fatherless, and the widow" (Deuteronomy 26:12).