In a governmental system, a party leader acts as the official representative of their political party. The party leader is typically responsible for managing the party's relationship with the general public. As such, they will take a leading role in developing and communicating party policy, especially election platforms, to the electorate. They are also typically the public face of the respective party and the principal media contact.
In many parliamentary systems, party leaders compete directly for high political office. It is thus typical in such states (e.g., in the Westminster system, which gave rise to the Prime Minister of the United Kingdom) for the party leader to seek election to the legislature, and, if elected, to simultaneously serve as the party's parliamentary leader.
In light of this last fact, individuals who became Prime Minister of their country need not be listed here, excepting for extraordinary circumstances.