The Sanhedrin was a Jewish religious body that was the high court of interpreting Jewish religious laws. At the time of Jesus the Roman authorities allowed them to exhibit considerable political, cultural, religious influence, as well as having their own temple tax of the people.
The Sanhedrin was made up of Pharisees and Sadducees. The two groups did not necessarily get along, but tolerated each other for their greater position. They were made famous in history for their opposition to Jesus and his ministry. Caiaphas was the high priest at the time of Jesus' ministry, having replaced his father Annas. Both questioned Jesus and, along with the rest of the Sanhedrin, brought about his eventual death. Not all members of the Sanhedrin were against Jesus. Nicodemus and Joseph of Arimathea were both members of the Sanhedrin who secretly supported Jesus.
With the destruction of Jerusalem in 70 A.D. after the unsuccessful Jewish revolt against Roman authority, the Sanhedrin as it had been known was put to an end. Other 'Sanhedrins' were established in various cities, but they were never the same.