Pharisees, Sadducees, Essenes, and Zealots were the four primary religious/political factions of the time.
Pharisees were keepers of the Law and held the entire (what we would call) Hebrew Bible as the word of YHWH. They emerged from the exile as the dominant faction because they (correctly) connected Israel's abandoning of the Law as the reason for the punishment of exile. As such, they created "fences" to attempt to keep people from even coming close to replicating this behavior and casting Israel into exile and further punishment.
Sadducees were more affluent and were also more sympathetic to the Hellenistic movement. They acquiesced quite a bit to the influence of the prevailing powers (Greece, and then Rome) because they realized it was economically and politically advantageous for them to do so. They also only held the Pentateuch as their authoritative Scriptures.
Essenes held themselves to a higher standard of piety - including voluntary poverty, abstinence, and other forms and degrees of asceticism. Additionally, they lived in a tighter community (Jerusalem had an "Essene Quarter") and may have influenced the early Christian community (of Acts 1-11). Some of them took a more radical approach on this communalism and established the community of Qumran.
Zealots were just that. They believed that change could only be affected in the ruling powers through force, and likely had not real religious leader.