It is not permissible to just connect the positive terminal to the negative terminal of a cell with a copper wire. It will overheat and get damaged. This happens because the electromotive force (emf) drives a large current through the cell because of the small resistance of the materials that make up the cell itself. This small resistance is called internal resistance, r and Ir = internal resistance drop. In this case the cell is said to be short circuited since the current is flowing through a path of least resistance because there is no load.
When a cell is connected to a bulb or any device with a significant resistance say a resistor, the emf or voltage is now distributed over the internal resistance, r and the external resistance, R. In this scenario: emf = Im .r + Im .R where I m is the main current = current that flows through the cell and the voltage drop across the external resistance or load is lower than the nominal voltage (emf) of the cell and this lower voltage is called terminal voltage.