What is EMF?

EMF:- The energy per unit charge that is converted reversibly from chemical, mechanical, or other forms of energy into electrical energy in a battery or dynamo. 

Terminal Voltage:- 
The voltage at the terminals connected to the source of electricity for an electric machine.

When no current is drawn from a cell  i.e. when the cell is in open circuit the potential difference between the terminals of the cell is called its emf.    
 
  when current is drawn from a cell i.e. when the cell is in the closed circuit the potential difference between the electrodes of the cell is called its terminal voltage.