Biblical Laws of Tzedakah

Leviticus
25:35
When your brother becomes impoverished and loses the ability to support himself in the community, you must come to his aid. Help him survive, whether he is a proselyte or a native [Israelite].

Deuteronomy
15:7
When, in a settlement in the land that God your Lord is giving you, any of your brothers is poor, do not harden your heart or shut your hand against your needy brother.

15:8
Open your hand generously, and extend to him any credit he needs to take care of his wants.

15:9
Be very careful that you not have an irresponsible idea and say to yourself, 'The seventh year is approaching, and it will be the remission year.' You may then look unkindly at your impoverished brother, and not give him anything. If he then complains to God about you, you will have a sin.

15:10
Therefore, make every effort to give him, and do not feel bad about giving it, since God your Lord will then bless you in all your endeavors, no matter what you do.

15:11
The poor will never cease to exist in the land, so I am commanding you to open your hand generously to your poor and destitute brother in your land.

Certain 'levies' [Matanot] on agricultural produce grown in Eretz-Israel must be paid by the physical removal from the produce of certain amounts to be given to certain people. There were basically two kinds of levy: Matnot Aniyyim and Matnot Kehunah - levies to be given to the destitute and levies to be given to the priesthood.

A. Levies to be given to the Destitute:

1. Pe'ah: one corner of each field had to be left unharvested by the owner. It was the privilege of the destitute to enter the field when it had been harvested and to harvest for themselves what had been left in the Pe'ah [corner]. The Torah itself leaves the amount to be left up to the generosity of the owner, but the sages set the average at around 2 percent of the whole. The poor did their own harvesting of the Pe'ah to avoid the impression that what they got from the field was the result of the farmer's personal generosity: it is their privilege granted them by God, who is the real owner of the field - and not the gift of the farmer.

2. Shikhechah: after the produce had been bundled into sheaves and so forth it was loaded onto carts for transportation to the granary. Any sheaves or bundles that on account of an error were not loaded could not later be reclaimed by the owner: the forgotten bundles [Shikhechah] had become the privilege of the destitute.

3. Leket (called in the vineyard and olive orchard Peret): the destitute had the right to follow after the harvesters and pick up anything that accidentally fell from their hand. (This was what the biblical Ruth did in the field of Boaz.)

4. Olelot: underdeveloped grapes that were harvested in error became the privilege of the destitute.

5. Ma'aser Ani: In every seven-year cycle [Shemittah] there was a levy of ten percent of what remained of the produce (after all the above 'deductions') in the third and sixth years of the cycle. This produce had to be given to the destitute person of the owner's choice.