"And concerning these things it is that great disputes and differences have arisen among them, while the Sadducees are able to persuade none but the rich, and have not the populace obsequious (obedient or giving deference) to them, but the Pharisees have the multitude on their side." (Antiquities of the Jews, Book 13, Chapter 10)
While their rivals received backing from the rich and elite, the Pharisees were popular with the people, who considered them the highest religious authority. They were especially influential in local synagogues. Jesus referenced their authority when he said they sat in "Moses' seat" (Matthew 23:2).
The New Testament Greek word for Pharisees is Pharisaios (Strong’s Concordance #G5330). It comes from the Hebrew word parash (Strong's Concordance #H6567) which means, "to separate or disperse" or "separated ones." They separated themselves not only from the heathen but also from other influences that regularly came into Palestine. They advocated and adhered to a scrupulous observance of both the written and oral laws as interpreted by Scribes, in accordance with tradition.
The Pharisees also believed that in addition to the written Torah (the first five books of the Old Testament) both they and the Sadducees recognized there existed yet another Torah. This "other" Torah was the Oral Torah, Oral Tradition, or "traditions of the elders" (Matthew 15:1 - 2). It comprised interpretations of the law that they thought God verbally gave to Moses but which were not included in the written version of the law. They asserted that the written Torah was not complete in and of itself and needed the Oral Torah.
The New Testament mentions several traditions practiced by the Pharisees. They fasted often (Matthew 9:14, Luke 18:12), and ceremonially washed their hands often, especially before eating bread. They believed "unwashed" hands were defiled (Matthew 15:1 - 2, Mark 7:1 - 4). They washed of cups, pots, pitchers, tables, etc. often (Mark 7:4).
Pharisees held to the belief that contributions dedicated to the temple (Corban or a gift to God - Mark 7:11) were more important and took precedence over helping their father and mother (the 5th commandment). The Pharisees even considered plucking an ear of corn to eat on the Sabbath as 'work' and breaking the fourth commandment (Matthew 12:1 - 2).
Pharisaic leaders were called "rabbis" or "teachers" (Matthew 23:6 - 7, John 3:1 - 10) and often attracted "disciples" to themselves (Mark 2:18; Matthew 22:16; Luke 5:33). Some of them were members of the Sanhedrin, the supreme council of the Jews that had jurisdiction over Jewish matters (Acts 5:34).
The Gospel accounts portray the Pharisees as opposing both John the Baptist (Luke 7:30) and Jesus (Matthew 12:38 - 39, 15:12; John 7:47 - 48). They also, early in Jesus' ministry, plotted with the Herodians to kill him (Mark 3:6). Not all of them, however, were against the gospel or opposed it all of the time. Some of them, like Nicodemus, were favorable to Jesus and believed in his message (John 3:1 - 2, John 7:50). The early church was composed of many of them (Acts 15:5).
Some of Jesus’ harshest criticisms given during his ministry were against those who adhered to Pharisaism. This was likely due to their popularity with the masses and that most people considered them the highest religious authority (as opposed to the Sadducees).