The Rabbis developed an elaborate system of tithing produce, mostly to provide livelihood for priests and Levites.
Reprinted from The Jewish Religion: A Companion, published by Oxford University Press.
The main biblical passages regarding the tithing of produce are: Numbers 18:21-32 and Deuteronomy 14:22-7 and 26:12.
Biblical scholars have seen the differences in these sources concerning the recipients of the tithe as due to the social background of two separate sources, each having its own applications. Throughout the Rabbinic literature, however, the sources are harmonized and the following system emerges.
The tithes have to be given from corn, wine, and oil by biblical law and from fruit and vegetables by Rabbinic law. The farmer first separates from the yield a portion (a sixtieth, fiftieth, or fortieth at the farmer's discretion), known as terumah ('heave offering' or 'gift'). This is given to a Kohen (priest) and is treated as sacred food in that it must not be eaten when the priest is in a state of ritual contamination or when the terumah itself has suffered contamination. Nor may it be eaten by a non-Kohen.
Three Kinds of Maaser
A tenth of the remainder of the yield, known as maaser rishon, 'the first tithe,' is then separated and given to a Levite. The Levite, in turn, separates a tenth of his tithe and this, known as terumat maaser, is given to a Kohen to be treated with the same degree of sanctity as the original terumah, The portion given to the Levite has no sanctity and may be eaten by an ordinary Israelite.
The farmer separates a tenth of the reminder of his yield, known as maser sheni, 'the second tithe.' This has to be taken to Jerusalem and consumed there in a spirit of sanctity. If it is too difficult to take the second tithe to Jerusalem, it can be redeemed by substituting for it a sum of money which is then taken to Jerusalem and food and drink purchased with it to be consumed there.
However, every third and sixth year of the cycle culminating in the Sabbatical year, the second tithe is given to the poor and is known as maaser ani, 'poor man's tithe.'
After the destruction of the Temple, maaser sheni was redeemed for a small amount and this tithe could then be consumed by the farmer wherever he happened to live.
Produce from which the tithes had not been separated was strictly forbidden but once the tithes had been separated their actual distribution could be postponed. The farmer could please himself as to which Kohen or Levite he gave his tithes.
Not everyone was scrupulous in separating the tithes. A whole tractate of the Mishnah, tractate Demai ('Doubtful Produce') is devoted to the need for tithing produce bought from an am ha-aretz suspected of laxity in the matter.