Like the other Jewish sects of its time, the Pharisee sect of Judaism was based on the observance of the Torah, with its rites, ceremonies, and insistence of the validity of the traditional interpretation of the Law of Moses.
In contrast to Saduccees, who insisted that the text of the Torah be followed literally, the Pharisees held that the books of the Tanach (Hebrew Bible, also called the written law) have always been transmitted in parallel with an oral tradition. They pointed as proof to the text of the Torah itself, where many words are left undefined, and many procedures mentioned without explanation or instructions; this they argue means that the reader is assumed to be familiar with the details from other, oral, sources. This parallel set of material was originally trasmitted orally, and came to be known as the "the oral law". By 200 CE much of this material was edited together into the Mishnah. Thus, from the Saduccee and Essene point of view, the Pharisees were the liberal party, which allowed for flexibility in the interpretation of the law.
This sect was present in the days of Jesus. Christians have traditionally seen Jesus as an opponent of the Pharisees, accusing them of being only outwardly religious, rather than inwardly observant of the Law. Jesus was opposed to the Pharisees emphasis on observance of religious purity laws. Some modern day scholars argue that this reading is no longer tenable, and that when the New Testament is read in its historical context, Jesus's attitude towards the law was more like a liberal or reform offshoot of Pharisee thought.
While during the first century CE and earlier, the Pharisees were faced with opposition from other Jewish groups such as the Essenes and the Sadducees, they were eventually triumphant; rabbinic Judaism as it is known today is descended from them.