What is the difference between 'emf' and 'terminal voltage' of a cell? Two cells of emfs E_1 and E_2 and internal resistances r_1 and r_2 are…

What is the difference between 'emf' and 'terminal voltage' of a cell ? Two cells of emfs $\displaystyle \mathrm{E}_{1}$ and $\displaystyle \mathrm{E}_{2}$ and internal resistances $\displaystyle \mathrm{r}_{1}$ and $\displaystyle \mathrm{r}_{2}$ are connected in parallel. Derive an expression for the emf and internal resistance of the equivalent cell.