Overview
A teller is a person who counts votes in an election, vote, referendum or poll. Tellers are also known as scrutineers, poll-watchers, challengers or checkers.
They should be distinguished from polling agents and counting agents who officially represent candidates.
United Kingdom
In the United Kingdom, tellers work on behalf of political parties (usually as volunteers). They stand or sit outside the polling station and collect electoral registration numbers (poll numbers) of voters as they enter or leave. They play no official part in the election and voters are under no obligation to speak with them. They are not polling agents, so they have no official rights, such as to enter the polling station. If asked, the tellers must explain they are not officials and why they are collecting poll numbers.
From Wikipedia (CC BY-SA 4.0).