The Pharisees were the largest group of “teachers of the Law” in the first century which is to say that they were teachers of the Law, but not all teachers of the Law were Pharisee. There were several other groups, most notably the Sadducees. These sects each represented a somewhat different theological tradition within Judaism, and I suppose it would be fair to compare them to modern day denominations in the sense that all were Jewish, but they taught things somewhat differently.
In the Gospels, the Pharisees are cast in a rather negative role as self-righteous, money loving, power hungry hypocrites, and while Jesus commented on these things often enough, we should probably recognize that not all Pharisees were quite so corrupted.
The main distinction between the Pharisees and other groups of scholars in those days was the singular devotion of the sect to keeping and maintaining the Law of Moses. Their devotion to keeping it was so great that in order to avoid even coming close to breaking the Law, they developed an oral law containing over two thousand additional rules to be memorized, and as a practical matter, annotated into the Law, so as not to even come close to a violation. Thus, since keeping the Sabbath was rather vague in the Law itself (you shall not work on the Sabbath), they came up with hundreds of additional rules so as to define anything that might be interpreted as “work”. As you know, this is the area into which Jesus boldly strode and conflict resulted several times.
Another example that comes up in Acts is their rule about only giving 30 lashes in punishment where the Law actually allowed 40. The Pharisees believed that rather than accidentally breaking the Law if there was a miscount, taking 10 lashes away from the maximum; they could be safe from making a mistake.
In addition to their study of the Law, the Pharisees also were serious scholars of the rest of the Old Testament. For this reason, they ran into serious conflict with the Sadducees who only accepted the Torah (Law) and not the rest (Talmud) of the OT. Thus, the Pharisees believed in a spiritual realm and a resurrection of the dead, which the Sadducees rejected.