What is "harisees"?
Pharisees were one of the leading sects within Judaism. Pharisees took pride in trying to adhere not only to the laws in the Old Testament, but a dizzying array of man-made laws as well. They were respected by the other Jews and enjoyed social, religious and political power in the community.
What is meant by Nicodemus being “ ruler of the Jews”?
This means that Nicodemus was one of the elite 70 or so members of the Sanhedrin, the political-judiciary-religious body that ruled Israel.
Didn't the Roman rule Israel at this time? And wasn't there also a king named Herod?
Yes, the Sanhedrin ruled under the supervision of Herod, whom the Roman emperor had appointed for the region that included Israel. Herod in turn was under the rule of the Roman emperor, who was represented by the Roman governor and legion stationed in the region.