What was the Sanhedrin?:
Sanhedrin comes from the Greek synedrion, which literally means sitting together and refers to a council of leaders political, religious, state, etc. Ancient sources have references to both Roman and Jewish Sanhedrin in Palestine, though most today associate the body with a supreme court of Jewish chief priests in Jerusalem.
When did the Sanhedrin exist?:
The Sanhedrin of Jerusalem existed in various forms over the course of several centuries. The Mishnah describes a Sanhedrin in operation during the Second Temple Period, between the fifth and fourth centuries BCE.
Where did the Sanhedrin operate?:
Many different Sanhedrin operated in hellenized regions, but the principle Sanhedrin that people are familiar with operated in Jerusalem.
What did the Sanhedrin do?:
The Sanhedrin of Jerusalem is believed to have been composed of chief priests and/or political leaders. It met to decide important cases and rule on disputed points of religious law. Its likely that the composition and activities varied over the several centuries it existed. It rarely used the name sanhedrin; instead, it used the Hebrew term for court: bet din. Its also likely that they had some sort of Roman oversight Rome was very good at using local elites to reinforce their control.
Why was the Sanhedrin important?:
The Sanhedrin is best known today for how it is portrayed in the gospels as trying and condemning Jesus to death. Whether such an event is even plausible is a matter of some dispute and the details of the gospel accounts are unlikely to be accurate.