Pharisees
So called from a Hebrew, word meaning "separate." After the ministry of the post-exilic prophets ceased, godly men called "Chasidim" (saints) arose who sought to keep alive reverence for the law amongst the descendants of the Jews who returned from the Babylonian captivity. This movement degenerated into the Pharisaism of our Lord's day -- a letter-strictness which overlaid the law with traditional interpretations held to have been communicated by Jehovah to Moses as oral explanations of equal authority with the law itself.
Compare (Matthew 15:2-3); (Mark 7:8-13); (Galatians 1:14).
The Pharisees were strictly a sect. A member was "chaber" (i.e. "knit together,") (Judges 20:11) and took an obligation to remain true to the principles of Pharisaism. They were correct, moral, zealous, and self-denying, but self-righteous (Luke 18:9) and destitute of the sense of sin and need (Luke 7:39). They were the foremost prosecutors of Jesus Christ and the objects of His unsparing denunciation (for example); (Matthew 23:13-29); (Luke 11:42-43).
Sadducees
Not strictly a sect, but rather those amongst the Jews who denied the existence of angels or other spirits, and all miracles, especially the resurrection. They were the religious rationalists of the time (Mark 12:18-23); (Acts 5:15-17); (Acts 23:8) and strongly entrenched in the Sanhedrin and priesthood; (Acts 4:1); (Acts 5:17). They are identified with no affirmative doctrine, but were mere deniers of the supernatural.