We have I guess different types of clergy.
Priests - that is, those of Aaronic patrilineality are tasked with performing certain religious services mostly in the Temple, on behalf of the nation and at times of the individual. What they can't do is determine how those services are performed.
Then there are Levites - those of the Levitic tribe excluding the Aaronic line- who are tasked perform as assistants to the Priests. They can't perform the services that Priests do, but they have their own designated functions in the Temple.
Then we have judges who are technically the closest to true Rabbis we have today. They deal with determining how religious functions and services should be performed, what precisely is prohibited, etc.
Then we have another class of Rabbis who are technically teachers of the entire body of Torah (written and oral).
Of the last two either one might lead a congregation (or even the wider general public). The former out of authority and the latter out of knowledge of authoritative opinions.
Neither of these Rabbis can perform priestly or levitic functions unless they happen to be either priests or levites.
There's also a cantor which is not necessarily a designated position. It's usually someone in the congregation with a nice voice for leading prayer services. Similar to that is the guy who reads the weekly Torah portion, although that requires memorization of the proper cantellation for each word as there are no cantellation marks in a Torah scroll.