A pollster is a person who surveys voters to find out who they're voting for or to gather other information about their opinions.
Often, pollsters attempt to predict the outcome of an election by polling people either before they vote or just afterwards. Pollsters sometimes also ask people questions about their opinions on social and political issues. Pollster, coined in 1939, comes from poll, "to take votes" or "to survey," and originally, "head of hair." This meaning was first extended from "head" to "person," and then in the 1620s to "counting heads."