A Christian children's song has a verse, "I don't want to be a Sadducee, because they're sad you see..." The song points out the key aspect of Sadducees that sticks out to us--they did not believe in any afterlife.
Your typical New Testament survey class usually suggests a few other characteristics: priestly types, didn't believe in angels or spirits, only believed the Law was Scripture, Roman collaborators. Some of these stock characteristics are actually highly debated once you enter scholarly circles. Several of them, for example, are based on single statements in single sources whose interpretation is highly debatable.
For example, the idea that Sadducees didn't believe in angels or spirits comes pretty much from a single statement in Acts 23:8: "The Sadducees do not believe in resurrection, neither of angel nor of spirit. But the Pharisees confess both." Needless to say, it sure sounds like angel and spirit here expand in some way on the word resurrection. Acts thus seems only to be saying that Sadducees did not believe in life after death.
The idea that the Sadducees only followed the Jewish Law and not the rest of the Jewish canon comes largely from a single statement in Josephus book 13: "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. "
But this passage is not contrasting the Law with, say, the Prophets or the Writings. It is contrasting some alleged following of the Law alone versus following the Law as interpreted by the "traditions of the elders." In reality, of course, we know that there never is some following of the Law alone. This is simply the normal perception of a religious group that they are following the Bible alone while other groups are following human traditions. In reality we all are following interpretive traditions.
The passage above goes on to suggest that the main following of the Sadducees was among the rich, and Josephus' statements that they were not a cohesive group probably indicates that they were not so much idealogicalically bound as socially bound, that is, what defined them as a group was primarily the fact that they were from a certain, somewhat aristocratic, priestly class of family.
The name "Sadducee" is close enough to "Zadok" to suggest a theory, namely, that when the Syrian king Antiochus Epiphanes took the priesthood away from Onias III in the early 2nd century BC, and then later when the Maccabees took over the priesthood (152BC), the displaced priestly families who were the true "sons of Zadok" continued to exist disempowered, but still wealthy and still in connection with the temple. They would represent a somewhat older strand of thinking, a more "conservative" one in the sense of resisting new ways of thinking like the newly risen apocalyptic stream of thought that arose in the early second century. Their rejection of an afterlife was simply the perpetuation of a point of view that was long standing at the temple, as we find in the Psalms. Sirach also seems to come from this social strand at the turn of the second century and embodies a similar view.
When the Romans took power in 63BC and then in 37BC when Herod the Great took over Jerusalem, the Sadducees were able to reinsert themselves into priestly leadership. We thus take largely at face value Acts 5:17 at face value to indicate a close association between many high priests and the Sadducean sect. They were Roman collaborators only insofar as it is good to be in power and when you can only be in power by maintaining a good relationship with those who grant you power, you walk a fine line.
The relationship between the Sadducees and the Essenes is an interesting one, as in some places the Dead Sea Scrolls take positions more similar to the Sadducees than the Pharisees. My hypothesis, one I haven't published and please remember me when you come into your footnotes, is that the Teacher of Righteousness was one of those displaced priests (here following James VanderVam's suggestion that the TR might have been the anonymous high priest of the years 159-152BC). But the group that would become the Essenes, "Enochic Judaism," I would suggest, was already in existence as an apocalyptic group well prior to this point.
The group we call the Essenes was thus a hybrid mixture of apocalyptic Judaism with a highly charismatic Sadducee. Even here, we are not talking about the Dead Sea group, which itself was a later, ultra-conservative spin off of this group that did not settle at the Dead Sea until some 50 years after the "Covenant of Damascus" was made, the settlement not existing until around 100BC.
And those are Schenck's thoughts on the Sadducees and Essenes, not likely to be developed in publication any time soon.