The emf of a cell is always greater than its terminal voltage. Why?

The emf of a cell is greater than its terminal voltage because there is some potential drop across the cell due to its small internal resistance. When a current flows through the cell, there is a voltage drop across the internal resistance, reducing the voltage available at the terminals (the terminal voltage). The electromotive force (emf) represents the total potential difference generated by the cell, while the terminal voltage is the potential difference actually available across the external circuit.