The Pharisees were a legalistic and separatistic group who strictly kept the law of Moses and the unwritten “tradition of the elders” (Matthew 15:2). They were by far the most influential of the ruling groups (Pharisees, Sadducees, and Essenes). It is believed this sect was formed out of the Hasidim (faithful ones), which was a broad movement in the second century BC that sought to preserve ancient Jewish tradition in the face of Hellenism. The name first appears around 135 BC.
They were found everywhere in Palestine by the time of Jesus and wore a distinguishing garb so they could be easily recognized. According to Josephus, they numbered around 6000. Their goal was to preserve Judaism, which developed into national pride and a feeling of superiority over others as seen with the Samaritans.
They believed they were the only interpreters of God and His word. As a whole, the Pharisees were seen by most other Jews as paragons of virtue and were highly regarded.
Their beliefs were often opposed to Jesus. They believed in final rewards for good works and a special divine providence for them. In the New Testament, the Pharisees are painted as purely evil. However, most Jews respected them and some converted (Nicodemus and Paul that we know of). Their goals and intentions were true; they just strayed a bit. With the fall of the temple in 70 AD, the Pharisees continued to lose power and were eventually replaced by the rabbis.
The Sadducees were more politically minded and had theological difference with the Pharisees including denial of the resurrection, angels, and spirits. The Sadducees were the political party of the Jewish aristocrat priesthood. They were priests, but not all priests were Sadducees. Under the Romans, they became the party favorable to the government. As aristocrats they were naturally very conservative and were more interested in maintaining the political status quo than in the religious purity of the nation. They were not popular with the people. Being satisfied with the present age, they did not care for the coming Messianic age.
The Sadducees rejected the tradition of the elders, denied the resurrection of the body, denied the existence of angels and spirits, and denied the existence of divine providence.
They seemed to have ignored Jesus’ early ministry, but feared a messianic movement would bring them political ruin. They sat in the Sanhedrin, which condemned Jesus. They were active against the early church and arrested Peter and John. They were responsible for the death of James, Jesus’ brother. However, with the destruction of Jerusalem in 70 AD, they disappeared as well. Hence, their struggle against Jesus, along with the Pharisees, resulted in demise anyways. No one can thwart God’s plans.
(Most summarized from Zondervan Illustrated Bible Dictionary by Douglas and Tenney).