The Pharisees were a Jewish political party which began in about 140 BC after the Maccabean revolt. After the destruction of the temple in AD 70, the beliefs of the Pharisees became the basis for Rabbinic Judaism from which comes traditional Judaism today.
The Pharisees were notable for their conflicts with the rival party of the Sadducees, and their differences helped define the views and beliefs of the Pharisees. The Sadducees tended to be wealthy and aristocratic, while the Pharisees represented the common people. The Sadducees favoured hellenization while the Pharisees resisted it. The Pharisees emphasised personal righteousness and adherence to the law while the Sadducees emphasised the importance of the temple (which they controlled). The Sadducees only recognized the first five books of the Bible (the Torah), while the Pharisees accepted all of the Hebrew Bible and many Rabbinical interpretations in addition to the written texts. The Sadducees did not believe in the resurrection or in angels, while the Pharisees did (Acts 23:8).
Many Pharisees became Christians (Acts 15:5) including Paul of Tarsus (Acts 23:6; 26:5).
The word “Pharisee” means “separated” and highlights their attitude in keeping separate from anything that would defile them.
Despite the love of the Pharisees for God’s word and their desire for purity, they became obsessed with man-made rules (e.g., Mark 15:1-9) and legal details while neglecting some of the important principles (Matthew 23:23). Consequently, Jesus singled them out for severe criticism (notably in his speech in Matthew 23).
For further information, see the Wikipedia article on Pharisees.