Introduction to Tractate Shekalim
In Exodus 30:11:16 God tells Moses that when he counts the children of Israel, he shouldnt do a simple head count but rather each person should donate half a shekel. The simple meaning of these verses is that the half-shekel is a one time donation, or at most a donation that is given only when a census is taken.
However, the rabbis understood the half-shekel to be an annual donation given to the Temple for the Temples upkeep, and for the purchase of the daily tamid and other offerings. These offerings had to be paid for with public money. Individuals could not buy them in order to donate them to the Temple. It is possible that the custom of collecting an annual tax from every Israelite already existed in the First Temple period (see II Kings 12:5 and II Chronicles 6:9; see also Nehemiah 10:33, where they give a third of a shekel). We should note that the Sadducees vehemently disagreed with this halakhah. They held that an individual could donate the tamid offering and they denied that there was a yearly half-shekel tax.
The value of a half-shekel might vary from time to time, because the half-shekel which they are referring to is a Torah half-shekel. In the Mishnah they assume that a shekel is worth a sela, which is four dinars (a Roman currency). Therefore, half a shekel is assumed to be two dinars. This two dinars is usually called a shekel. Hence, throughout the tractate when the mishnah says shekel it is referring to the half-shekel (I realize that this is confusing).
According to the Mishnah, they began to remind people to donate the half-shekel in the beginning of the month of Adar. This is where the custom, observed to this day, developed to read Parshat Shekalim, the above mentioned passage in Exodus, on the first Shabbat of Adar of every year.
Our tractate deals with many of the details of these laws, how the half-shekel was collected, what was done with it and who handles Temple funds. There is a lot of information in this tractate about the financing of the Temple, so perhaps it will be interesting to any accountants out there! The Babylonian Talmud does not contain a commentary on this tractate, as it does for all of the other tractates in Seder Moed. This is assumedly because none of the tractate was practically applicable after the destruction of the Temple. However, there is commentary in the Palestinian Talmud (the Yerushalmi).
Good luck learning Shekalim!