This builds on Frank Luke’s answerFrank Luke’s answer, especially Characteristic #4 (“This was a lay order of teachers”).
The Pharisee interpretation of the priestly Temple service was that it serves as a symbol and an ideal of Man’s duties to God, rather than being that duty. Along with a focus on the study of the Law came an effort to incorporate the lessons from Temple service into daily life. This change of focus allowed the Pharisee way of life to survive the destruction of the Temple.
Their defining characteristic was chulin al taharas hakodesh, everyday life conducted with the purity required for the sacred. Just as the priestly portion could only be eaten in a state of ritual purity, so too did the Pharisees (wherever possible) eat their daily food in such a state. Similar to the way the priests washed/purified/sanctified their hands and feet before Temple service, Pharisees ritually washed/purified/sanctified their hands before prayer, study, or even eating. (Washing before eating in particular was so widely accepted by the followers of the Pharisees among the common-folk, that anyone neglecting this was assumed to be non-Jewish, or at least grossly non-observant.)
(The preceding explanation is adapted from Rabbi Samson Raphael Hirsch’s commentary to the Biblical passages regarding the Temple service.)
Observance of many Pharisee ordinances was easily visible, and could be imitated by people who otherwise ignored the more central aspect of the Law. As Alexander Yannai I told his wife Shalomtzion,
“Have no fear of the Pharisees nor the Sadducees. Fear the hypocrites who pretend to be Pharisees; they act like Zimri and demand reward like Pinchas.” (Sotah 22b.)
The Pharisees themselves categorized several species of this hypocrisy, including the sort who make a show of their alleged piety or claim to have fulfilled their duties—these they called “a plague of Pharisees” and classified as “those who bring destruction to the world”. (Ibid. 20a–22b)
(A cheap propaganda trick thus available to rabble-rousers was to broadly accuse all Pharisees of this hypocrisy, making the audience of common-folk feel superior.)