These were two very different groups. They can be seen as sects of Judaism, but their differences go beyond religious belief. There were important differences between them in terms of demographics, politics, and culture.
Basically, the Sadducees were the more cosmopolitan elites. They were more willing to accept the Greek/Hellenistic culture that had come to Palestine with Alexander the Great. Politically, they tried to reach accommodations with the occupiers of their land. In religious terms, they were very oriented towards ritual, emphasizing the temple and the rites that occurred there as the most important aspect of their faith. They accepted the authority of the written Torah, but did not accept the authority of oral traditions.
The Pharisees were the opposite of all of these things. They were not from the elite classes. They were more traditional in their culture, not Hellenized. They did not get along with secular authority as well. They believed in the authority of the traditions, not just the written Torah.
Thus, these were groups that were very different in a number of ways.