Shekel (or sheqel), is an ancient Mesopotamian coin, usually of silver. It was based upon a unit of weight, around 9.6 or 9.8 grams and used in Bronze Age Europe for balance weights and fragments of bronze that may have served as money. It also became currency in ancient Tyre, Carthage and in Judea during Hasmonean times. 

According to Levitical law, whenever a census of the Israelites was to be conducted, every person counted was required to pay the half-shekel for his atonement (Exodus 30:11–16). During the Second Temple period, it was customary among Jews to annually donate a half-Shekel into the Temple treasury, for the upkeep and maintenance of the Temple precincts, and also for purchasing animals for public offerings. This practice applied both to Jews living in Israel and those living outside.

Tyrian shekels were coins of Tyre, which in the Roman period were used for payment for the Temple tax in Jerusalem.  The coins bore the likeness of the Phoenician god Melqart or Baal, derided as Beelzebub by Jews in the time of the Seleucids. Because Roman coinage was only 80% silver, the purer (94% or more) Tyrian shekels were required when paying the temple tax in Jerusalem. 

In Jesus' time, the money-changers and those selling sacrifical doves were all under the control of the Temple Priesthood. The priests had developed a policy of providing a poor exchange rate for people's Roman money, making the acquisition of the required Temple tax more difficult for pilgrims coming to the Temple. This, and the exhorbitant fees they charged for sacrifical doves was the reason for Jesus' ire and his description of them as "robbers."