Jesus often encountered "Pharisees" and "Sadducees". For example,
34 Hearing that Jesus had silenced the Sadducees, the Pharisees got together. 35 One of them, an expert in the law, tested him with this question: 36 “Teacher, which is the greatest commandment in the Law?” (Matthew 22:34-36)
From a Christian perspective, Who were they and how did they differ?
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.
The difference between the Pharisees and the Sadducees are mentioned by Jewish Priest Josephus and also New Testament.
The Pharisees - The Pharisees, the scribes, and everyone belonging to the sect of the Pharisees obeyed Traditions of the Elders. Traditions of Elders is also known as Traditions of our fathers (Galatians 1:14) or Traditions of our forefathers by Jewish Priest Josephus (Antiquities of Jews Book XIII.X.VI).
Jesus Christ severely criticized the Pharisees and the scribes for obeying Traditions of the Elders which nullified the word of God (Matthew 15, Mark 7). That's why the biggest opponents of Jesus Christ were the scribes and the Pharisees (Matthew 23, Luke 11).
Traditions of the Elders later came to be known as Talmud. This is also agreed by Jewish society.
Rabbi Michael Rodkinson - "The Talmud, then, is the written form of that which, in the time of Jesus, was called the Traditions of the Elders, and to which he makes frequent allusions" (Source - The History of the Talmud, Vol. II, page 70, Chapter IX).
The Sadducees - One of the major differences between the Pharisees and Sadducees is that the Sadducees rejected Traditions of the Elders (a.k.a Traditions of our forefathers).
Here is the information from Jewish Priest Josephus.
"What I would now explain is this, that the Pharisees have delivered to the people a great many observances by succession from their fathers, which are not written in the laws of Moses; and for that reason it is that the Sadducees reject them, and say that we are to esteem those observances to be obligatory which are in the written word, but are not to observe what are derived from the tradition of our forefathers." (Antiquities of Jews Book XIII.X.VI).
Although the Sadducees encouraged to learn the laws of God given through Moses and the rest of the Old Testament, still they never kept it. They had their own doctrine. In the doctrine of Sadducees, they rejected the existence of the spirits, angels, and resurrection while Pharisees acknowledged the existence of the spirits, angels, and resurrection (Acts 23:8).
That's why Jesus Christ tells his disciples to be beware of the doctrines of the Pharisees and the Sadducees (Matthew 16:12). Jesus Christ also tells the Sadducees how they don't know both the scriptures of God and the power of God (Matthew 22:8).
That's why we see Jesus Christ saying this.
John 5:46-47 (NIV) - "If you believed Moses, you would believe me, for he wrote about me. But since you do not believe what he wrote, how are you going to believe what I say?”
John 7:19 (NIV) - "Has not Moses given you the law? Yet not one of you keeps the law. Why are you trying to kill me?”
The answer from swasheck is pretty good, and more than I personally know. But I'll put in 2 cents just for fun and completeness.
The Sadducees denied bodily resurrection. Which is why, as any Catechist will tell you, is why they are "so sad, you see".
As for the Essenes, lots of folks speculate that John the Baptist was one of these folks (although since he was in the desert, who knows how that would affect him) and those same folks would probably say Jesus was too. But since Jesus has a divine intellect, I'm not sure that partisanship would change His way of thinking on anything.
