EMF of a cell is the electrical energy the cell supplies, converted from its chemical energy, per unit charge passing through the cell. Make sure you learn that definition and UNDERSTAND it, which I guess means that you will automatically have it memorised. That's a great reason to try and understand concepts rather than memorise blindly.
Note that a cell has a thing called internal resistance, so there is resistance to current inside the cell itself. Therefore energy is dissipated (as heat) in the internal resistance and this energy, per unit charge, we informally call the 'lost volts' (make sure you see why). Hence, by V=IR, and if we call the internal resistance r, the 'lost volts' is clearly Ir.
So this means, that if volts are 'lost', the pd that the components in the circuit receive is actually less than the emf! This pd will be E-Ir, where E is the emf.
We call this pd the terminal pd V, so that V=E-Ir (recognise that equation?). The terminal pd is defined as the electrical energy delivered to the external circuit and converted into other forms of energy per unit charge. Understand that too.
That's pretty much that for emf and the like. You'll be surprised how useful this concept is.